Melatonin and testosterone propionate (TP), either alone or in combination, were injected into groups of pregnant rats and into 2-, 6-, and 13-day-old female animals. Melatonin had an inhibitory effect on vaginal introitus and decreased the incidence of estrous smears, when injected during the early postnatal period. TP inhibited the canalization of the vagina in prenatally injected rats and caused precocious vaginal patency in animals injected on days 6 and 13. Female rats given TP alone showed persistent vaginal cornifîcation, while melatonin in combination with TP reduced the incidence of such persistent vaginal estrus and established irregular vaginal cycling.