Male rats castrated shortly after birth respond to testosterone injections in adulthood with an incomplete pattern of masculine mating behaviour (Beach & Holz, 1946; Gerall, 1958; Harris, 1964; Grady, Phoenix & Young, 1965) and display elements of female sexual behaviour more frequently than males castrated in adulthood (Harris, 1964; Feder & Whalen, 1965; Grady et al. 1965). A search was made for structural differences in the brain which might correspond to these persisting behavioural effects of neonatal castration. Four male rats were anaesthetized and castrated 7 days after birth and were then treated like their normal litter-mates, three males and two females. The nine animals were killed when 126 days old. Histological and sampling procedures were the same for all brains. Cross-sectional areas of the brain and brain structures were determined by planimetry on tracings of their magnified images. Also, cells were counted in some parts of the brain. The