Although chronic administration of physiological amounts of oestrogen alone is an effective means of inducing lordosis in female rats (Davidson, Smith, Rodgers & Bloch, 1968), progesterone reliably facilitates lordosis following acute or sub-maximal treatment with oestrogen (Boling & Blandau, 1939; Beach, 1942). In males, spontaneous lordosis occurs only rarely, and there are few reports on the successful induction of this behaviour by ovarian steroids (see Young, 1961). Recently, however, it has been shown that chronic administration of oestradiol benzoate to castrated male rats will result in lordosis response rates approaching in frequency those found in females (Davidson, 1969). Male rats castrated during the first few days of life, but not later, show behavioural responses closely resembling those of normal females after treatment with ovarian steroids in adulthood. This suggests that testicular secretion in the neonatal period is responsible for the determination of future behavioural patterns (Grady, Phoenix & Young, 1965;