Myometrial activity in vivo was quantified by video-laparoscopy in early pregnant rats given doses of RU486 which caused embryo abortion and blocked the action of progesterone on the vagina. Alltreatments diminished the frequency of circular contractions and abolished the curling movements ofthe uterus which are characteristic of pregnant, pseudopregnant and progestin-treated rats. The effectsof RU486 on circular contractions were similar in pseudopregnant rats, i.e. they were not a consequenceof embryo abortion. These results support the thesis that increased myometrial circular contractions inearly pregnancy and pseudopregnancy are induced by increasing levels of progesterone. Effects of RU486on longitudinal contractions were more complex: the highest dose inhibited longitudinal contractionson Day 5 of pregnancy and pseudopregnancy, but increased their frequency on Day 6. The acuteinhibition of longitudinal contractions by RU486 was unexpected and the mechanism remains to beelucidated. The later increase in the frequency of longitudinal contractions appears to be due toantagonism of progesterone by RU486. The frequency of caudal longitudinal contractions on Day 6 inmated rats given RU486 was similar to that in unmated oestrous rats, but the frequency of craniallongitudinal contractions was significantly higher. These results support the hypothesis that stimulireceived during copulation may have long-term effects on myometrial activity, by increasing pacemakeractivity at the cervix.