The capacity of oestrogen to activate sexual behaviour in castrated male rats has been reported repeatedly (see Davidson & Bloch, 1969; Pfaff, 1970). The mating pattern observed after oestrogen treatment is mostly incomplete, intromission and ejaculation patterns being infrequent or entirely absent. A possible reason for the deficiencies seen in the mating pattern of the castrated, oestrogen-treated males is an insufficient development of the penis since oestrogen does not stimulate normal growth of the accessory sex organs (Price & Williams-Ashman, 1961). The present communication reports a study in which castrated rats, given oestradiol benzoate (EB), were treated additionally with 5α-androstan-17β-ol-3-one (dihydrotestosterone, DHT), a metabolite of testosterone which is highly potent in stimulating penile and accessory sexual organ growth but has little, if any, stimulatory effect upon sexual behaviour (Feder, 1971; Whalen & Luttge, 1971). Male rats were castrated at 30 days of age and treated as follows: Group A rats