Unmated female rats received medroxyprogesterone acetate (MPA) on the day of oestrus. Myometrial activity in vivo was quantified by video-laparoscopy 24-96 h later. Changes induced by MPA resembled those in early pregnancy/pseudopregnancy, but were not as extreme. There was a significant increase in circular contractions, followed by a decline, as in mated rats. Increases in circular contractions propagating caudally were as large as in mated rats, but increases in those propagating cranially were much smaller and qualitatively different. MPA induced only small increases in the frequency of longitudinal contractions propagating cranially, whereas large increases occurred in pregnancy/pseudo- pregnancy. We conclude that in early pregnancy/pseudopregnancy, increased circular activity is induced by rising progesterone levels. However, progesterone alone does not account for the increased contractions propagating cranially in mated rats. We suggest that these are induced by increased pacemaker activity in the region of the cervix, possibly activated by stimuli received during copulation.