The purpose of the present work was to investigate whether the administration of a dopamine agonist (2-bromo-alpha-ergocryptine) was able to interfere with the normal change in uterine sensitivity to oestrogen that occurs around the onset of puberty in the rat. To determine the change in uterine sensitivity groups of rats at 25, 30 or 35 days of age were killed (to find the initial uterine weight) or ovariectomized with or without oestrogen replacement therapy (0.05ug/100 g body wt, daily for 5 days) and then killed to determine the final uterine weight. Comparisons were made between initial and final uterine weights. Five days before the animals reached 25, 30 or 35 days of age, one group was injected with 2-bromo-alpha-ergocryptine (0.05 mg/100 g body wt daily for 5 days) and another one with solvent (control rats). At the ages specified (25, 30 and 35 days of age) both groups were subjected to the ovariectomy schedule. Results showed that the bromocriptine treatment was effective in blocking the normal uterine change in sensitivity that occurs at 30 days of age. At 35 days of age the dopamine agonist was able to counteract the action of oestrogen in maintaining the uterine weight 5 days after ovariectomy. The results are interpreted as suggesting that the mechanism of change in uterine responsiveness to oestrogen in maturing rats is mediated by prolactin.