The phenomenon of male-induced, precocial puberty was investigated in CF-1 mice with particular regard to changes in plasma FSH and LH at various times after initiation of male exposure. A series of pilot experiments assessed procedural factors including the normalcy of ovulation and mating in such females. For comparative purposes, levels of FSH and LH were also determined during the adult estrous cycle and early pregnancy. Pairing of singly caged, immature females with adult males elicited estrual changes in FSH and LH but with considerable animal-to-animal variation in time after pairing. Uterine growth was detected in some young females as early as 24 h after male exposure began, and many females showed enlarging uteri by 36 h. Plasma LH concentrations did not change during this time period and circulating FSH, if anything, was progressively depressed for the first 48 h of male exposure. Proestrus-like changes in circulating LH and FSH were first detected in some young animals during the late afternoon of day 3 (60 h). Assuming that the observed uterine growth was in response to estrogen, the initial actions of the male on gonadotropin secretion in immature females may thus be too subtle for detection but still sufficient to result in enhanced estrogen secretion. Following this early period of presumed estrogenic activity, however, adultlike release of FSH and LH culminated in normal ovulation and mating.