Summary. Twenty female rats were implanted with myometrial electrodes and intrauterine balloons to study the effects of mating on uterine contractile activity. Mating produced both immediate and delayed effects. Uterine activity typically doubled during the period of active mating (the immediate effect), then briefly returned to the premating activity level after ejaculation. Beginning 5 min after the ejaculation, the delayed effect of mating was observed: contractile activity increased when the baseline activity had been low, but decreased when baseline was fast. This delayed effect lasted for 30 min to several hours. Vaginocervical stimulation seemed to play a critical role in these responses: artificial vaginocervical stimulation mimicked the response while vigorous mounting by males that were prevented from intromitting did not evoke these responses. Vaginocervical stimulation, whether mating-derived or experimenter-delivered, produced immediate and delayed effects on uterine contraction rate.