SUMMARYIn twenty oligospermic or azoospermic patients with elevated plasma FSH, the mean concentrations of plasma oestrone sulphate (843 ± 233 pg/ml), oestrone (54 ± 10±4 pg/ml) and oestradiol (46±6 ± 12±6 pg/ml) were found to be significantly higher than in twenty‐one normal fertile men of comparable age (593 ± 220 pg/ml, 40±6 ± 8±8 pg/ml and 33±1 ± 10±9 pg/ml respectively). SHBG binding capacity was elevated in the infertile group (infertile 3±35 ± 0±82 × 10−8M/l v. normal 2±76 ± 0±89 × 10−8M/l) but the total plasma testosterone concentrations were comparable (infertile 5435 ± 1578 pg/ml v. normal 5046 ± 1102 pg/ml). Evidence was cited to support the view that Sertoli cells, in response to an unphysiological FSH stimulation, are a likely source of excessive oestrogen production. The possible significance of increased intra‐testicular and circulating oestrogen in the altered state of testicular steroidogenic function in men with primary seminiferous