Vaginal stimulation (VS) in virgin female rats on the first day of estrous induces a significant increase in serum prolactin levels 15 min, 6,9,10 and 12 h after stimulation. Normal level values were observed 2 h after VS but a significant decrease appeared 8 h after VS. In nonstimulated rats a significant variation was observed, with a peak at 10.00 and 18.00 h and minimal values at 12.00 and 16.00. Adenohypophysial prolactin concentration showed a significant decrease both 15 min and 6 h after VS. A significant increase occurred 4 h after VS. At 8, 9, 10 and 12 h after cervical stimulation, the levels of prolactin were similar to levels in the control rats. The curve obtained in nonstimulated rats showed a significant decrease between 10.00 and 12.00 h; the rest of the curve showed no significant variation. The levels of prolactin in the hypophysis and serum during the first 12 h after VS clearly show a phasic release of the hormone and a correlation between synthesis and release, suggesting the existence of some kind of neural hormonal factor of regulation.