Summary. In about 60% of rats that were paired with fertile males before vaginal opening, vaginal opening occurred between 15:00 and 21:00 h under controlled lighting conditions (lights on 06:00–18:00 h). No rats mated before 12:00 h or after 03:00 h. Most of the rats (52/82) mated within 3 h after vaginal opening. About 90% of the rats that were paired with fertile males before vaginal opening mated and most conceived. Serum LH and FSH levels rose from 14:00 h to a peak at 18:00 h, whereas hypothalamic LHRH content suddenly decreased by 18:00 h.
This study shows that the timing of sexual receptivity, ovulation and the release of gonadotrophins during puberty are similar to those at pro-oestrus in adult rats and suggest that the diurnal rhythm of hormonal changes plays an important role in timing of the first oestrus.