Recent experiments (Carraro, Corbin, Fraschini & Martini, 1965) indicate that a single injection of 50 μg. reserpine given to female rats on the fourth day of life results in delayed vaginal opening, a decreased pituitary luteinizing hormone (LH) content at 30 and 60 days of age and prolonged periods of dioestrus. One aim of the present study was to test the effect of early reserpine treatment on adult behavioural cyclicity and mating behaviour of female rats. Following the report of Kikuyama (1961) that neonatally administered reserpine markedly attenuates the effects of oestrone on ovulation and vaginal cyclicity, we sought to determine whether treatment with reserpine prevents the syndrome of anovulation and loss of mating behaviour that occurs after treatment with a low dose of a more potent oestrogen (oestradiol benzoate). Female Sprague-Dawley rats were treated when 4 days of age (96–108 hr. after birth) as summarized in Table I. When