As an analogue of the juvenile hormone we used [2,3-3H-propenyl] -10,11 - epoxy - 6,7 - trans - 2,3 - trans - farnesylpropenyl -ether [designated as H]. The compound was labelled with 3H and had a specific activity of 28 Ci mmol1. Fig. 1 Changes in distribution of H in the epidermis (×) and the remaining body parts (O) of pupae of Tenebrio molitor L with time. Pupae were injected with 108 mol 11 labelled hormone in 1l insect Ringer solution. After defined time periods the epidermis was separated from the remaining body and both body parts homogenized in 50 mM Tris (pH 7.4). After addition of 1 ml methanol the undegraded hormone was extracted with 3 ml petrolether. The products of degradation remained in the aqueous phase. After evaporation under nitrogen the lipophilic phase was separated by thin layer chromatography on silica and the band of the hormone eluted with dioxane. The radioactivity of the hormone band and the radioactivity of the aqueous phase were counted in a liquid scintillation counter. By relating with an external standard 100% yield was calculated. All operations were at 4 C. The ratio (Q) of undegraded hormone to total radioactivity in % is plotted against time. We compared the decomposition rate of H in the suspected target organ epidermis with those in the lymph and all remaining organs taken together. H is accumulated in the epidermis while at the same time it is decomposed quickly in the lymph and in the remaining organs (Fig. 1). This demonstrates that the epidermis cells of Tenebrio molitor pupae form the main target organ for the juvenile hormone.Next we wanted to approach the molecular mechanism of H accumulation and reduction of H degradation rate in epidermis cells compared with the remaining organs. It was thought that H binds to a specific receptor in a analogous way to oestrogen receptor binding5. By tracing the radioactivity of bound and free H after separation on a 'Sephadex' column ('Sephadex' G200 fine), it is seen that H binds to a high molecular substance both in vitro and in vivo in homo-genates of epidermis cells which were washed free of lymph. H can be recovered from this binding almost quantitatively and unaltered by extraction with lipophilic solvents. The H receptor complex has a molecular weight of 360,0003%, as found by gel chromatography ('Sepharose 6 B' and 'Sephadex' G200 fine) in Tris-buffer (pH 7.4), containing 0.4 M KCl, 103 M EDTA, 1 vol. % 'Triton X100'. Its isoelectric point is 4.4 as determined by the method of Radola6. Under the conditions used (concentration of H and composition of buffer) only one single high molecular substance was found. Treatment with 6 M urea does not change the molecular weight of the complex. This indicates that the receptor is not composed of H-binding subunits. The specificity of the receptor is evident from its high affinity constant for H which is 4.4 × 10101 mol1, calculated from the results of gel filtration using the method of Scatchard7. This explains the molecular basis for the accumulation of hormone analogue in the target organ (epidermis) from which the receptor was extracted. In view of the low concentration of the receptor of 1.7 × 1010 mol 11 in a pupa we have not attempted to isolate it for further structural studies. We tried to obtain information on its nature by digestion with specific enzymes. Assaying the hormone binding of the receptor by gel filtration it was seen that DNase and phospholipase C do not have any influence whereas trypsin and RNase destroys the receptor complex. No H-binding activity smaller than receptor could be detected. RNA and protein are constituents of the receptor.The RNA content of the receptor suggests other functions as well as accumulation. This suggestion is supported by recently published results on oestrogen and androgen binding RNP particles of target organs of mammals8. These have a different molecular weight and therefore perhaps a different intracellular localization and functi