Myométrial activity was analysed by video-laparoscopy in unmated and mated oestrous female rats. In the former, most contractions propagated caudally and were predominantly longitudinal, with circular contractions limited to weak peristalsis. Mating had dramatic effects on activity, inducing a high frequency of strong peristaltic and non-peristaltic circular contractions propagating both cranially and caudally. Activity remained elevated for up to 10 h, decreasing to control levels as uterine contents were lost when the vaginal plug disintegrated. Mechanical stimulation of the cervix, sufficient to induce pseudopregnancy, increased the frequency of weak peristalsis but did not induce myométrial activity comparable to that seen after mating. Treatment of mated females with indomethacin returned myométrial activity towards control levels, suggesting that myométrial stimulation involved prostanoid production. After mating to intact males the uterus took on a characteristic blue colour suggestive of hypoxia. This too was abolished by indomethacin treatment. Removal of accessory reproductive glands from males showed that induction of myométrial activity and the colour change both required constituents from the vas deferens, seminal vesicles and coagulating glands. Males lacking seminal vesicles or coagulating glands were infertile. The prolonged myométrial activity and putative hypoxia induced by seminal constituents may be important in survival and selection of sperm.