In view of the significant role the sexual instinct plays in psychiatric illness, it is surprising that there are in the literature so few reports of attempts to suppress male sexual behaviour by the direct method of modifying the physiological basis of sexual activity by oestrogen therapy. Dunn (2, 3) established that administration of stilboestrol could result in marked reduction of male libido and sexual behaviour. He treated two non-psychotic hypersexual patients and, apart from the clinical effects, observed gynaecomastia, histological evidence of reversible testicular degeneration, and depressed urinary androgen excretion. Hamburger (6) later demonstrated in one case that oestradiol administration produced the same degree of urinary androgen excretion as did actual surgical castration of the same patient. Thus, present evidence indicates that oestrogen therapy produces in the male what is, in effect, a reversible chemical castration.