The use of 'Sesamex' is further illustrated in recent investigations of chlordene (1; Fig. 1) and related compounds. Heptachlor (2), which is known to be converted into a persistent epoxide, m.p. 160, in mammals4 and the housefly5, is considerably more toxic than chlordene (1) to the housefly. Stereochemical investigations of Riem-schneider6, chemical investigations of Davidow and Radomski4 and inspection of molecular models suggest structure (3) for this epoxide, while the mode of formation of a second epoxide, obtained indirectly from heptachlor7, indicates that it has the structure (4). Chlordene (1) interacts with peracids to give the epoxide (5) (ref. 8) and is converted by the housefly into an epoxide-like compound9 now identified as compound (5) (infra-red spectrum). Gas-liquid chromatographic analysis, on an 'Apiezon L'-treated 'Celite' column, of acetone tissue extracts of live houseflies treated with chlordene (peak I) revealed the presence of at least two other compounds (peaks III and IV in order of retention time) in addition to the epoxide (peak II). No substances other than chlordene were found in heat-killed insects similarly treated. Compounds corresponding to peaks III and IV were isolated from insect extracts by thin-layer chromatography on alkaline alumina and compound (III) was identified (infra-red spectrum) as a 1-hydroxychlordene (6), also prepared from the 1-bromo-compound (7) by alkaline hydrolysis. Examination of acetone extracts of houseflies treated with compound (5) or (6) revealed the presence, in either case, of a substance which co-chromatographed (in thin-layer chromatography and gas-liquid chromatography) with compound IV and suggested that this might be a hydroxy-epoxide such as compound (8), arising by epoxidation of (6), hydroxylation of (5), or simultaneous hydroxylation and epoxidation of chlordene. Perbenzoic acid oxidation of compound (6) then gave a hydroxy-epoxide (8) having an infra-red spectrum identical with that of compound IV isolated from chlordene treated houseflies. Although epoxide (5) might be hydroxylated in one or more of several positions, the fact that compound IV could not be resolved and the clarity of its infra-red spectrum suggested that it was mainly compound (8); if free isomeric mono-hydroxy compounds are formed the amounts present must be small. Compounds (6) and (8) were non-toxic when injected into houseflies. Acetone extracts of houseflies treated with epoxide (5) also contained compounds which may be chlordene-diols since their chromatographic behaviour was similar to that of synthetic chlordene glycols. The latter were clearly distinguished from the other compounds discussed since they remained near the point of application in most of the thin-layer systems used. Fig. 1. Chlordene and partial structures of compounds derived there- from by modification of the unchlorinated ring. Figures in square brackets are corresponding ?LD50 ?s in ?g per female housefly (ref. 2).LD50 following pretreatment with 'Sesamex' Although chlordene was recovered unchanged from heat-killed flies, rinses of the vessels which contained them showed traces of more polar compounds, and experience with similar compounds3 suggested that chlordene might undergo atmospheric oxidation. This was verified by the decomposition which resulted from several hours exposure of chlordene to air and light on glass plates. Further, chlordene exposed to ultra-violet light for a few minutes gave a complex mixture of products from which compounds (5) and (6) were isolated. These results indicate the susceptibility of the chlordene molecule to oxidative conversions.Since chlordene is converted in vivo into the epoxide readily obtained chemically, the structure?activity relationships in this group of compounds may be considered in detail. The apparent toxicities to the housefly of compounds (3), (4) and (5) decrease in this order (Fig. 1), the last-mentioned compound being only slightly more toxic than its precursor (1) (ref. 2). Since the in vivo epoxidation of heptachlor appears to be a toxication process5, it seemed that the relatively low toxicity of chlordene might result from its inability to form a stable epoxide in the housefly9. Penetration investigations with chlordene have confirmed that its rapid disappearance from housefly tissue is accompanied by a temporary accumulation of the epoxide, the levels of both compounds falling to zero within, for example, 8 h following application of 04 ?g chlordene per female fly. Fig. 2 shows the effect of a prior application of 'Sesamex' on the fate of chlordene epoxide (5), topically applied to the housefly. The discovery of ring hydroxylation as a detoxication route for chlordene epoxide helps to explain the stabilizing effect of 'Sesamex', an inhibitor of biological oxidations, on this epoxide in vivo, since the more obvious route involving conversion of chlordene epoxide to glycol(s) involves hydrolytic cleavage of the epoxide ring rather than oxidation. The stabilization of compound (5) in vivo is accompanied by an approximately ten-fold increase in its toxicity (Fig. 1), indicating a potentially higher toxicity than that observed in normal toxicity tests. In contrast, epoxides (3) and (4) are found to accumulate in the housefly at about the same rate following their topical application and both appear to be as persistent as diel-drin10. This finding accords with the lack of any pronounced synergistic effect with 'Sesamex' and indicates that the observed toxicities of these compounds approach their intrinsic toxicities (that is, toxi cities in the absence of detoxication). The intrinsic toxicity of chlordene epoxide appears to lie between those of the two heptachlor epoxides and may be greater than the maximum so far observed since the recoveries from insect tissue were incomplete, even in the presence of 'Sesamex'. The results suggest that the different toxicities of the heptachlor epoxides (3) and (4) may be a true consequence of their stereochemical difference, while the apparent low toxicity of chlordene epoxide results from an increased liability to detoxication arising from the absence of the additional chlorine atom present in (3) and (4). Rather than possessing a more positive role in toxicity, therefore, this chlorine atom may serve simply to shield a vulnerable position of the chlordene molecule from direct hydroxylation while also screening the epoxide rings from enzymatic attack. A toxicological investigation of the chlordene epoxide having its oxygen ring in the same configuration as that of compound (4) would be of interest in relation to this hypothesis. Fig. 2. Effect of pretreatment with 'Sesamex' (5 ?g/fly) on the recovery, determined by gas-liquid chromatography3, of chlordene epoxide from the housefly. Dotted curves, cuticle rinses; full lines, tissue extracts. Insecticide only, ? ; insecticide plus 'Sesamex', ?. Dose, 037 ?g/fly