To elucidate the role of corticotrophin-releasing factor (CRF) in the spontaneous rhythmic contractions of pregnant myometrium, isometric contractions of longitudinal myometrial preparations taken from Sprague-Dawley rats (18-21 days gestation) were measured. CRF (0·03-30 nM) significantly increased the amplitude (117 ± 4 % of control at 30 nM, mean ± S.E.M.) and significantly decreased the frequency (74 ± 5 %) in a concentration-dependent manner. The area under the tension curve did not increase (89 ± 8 %). Such effects of CRF were not observed in preparations pretreated with the RP diastereomer of adenosine cyclic 3',5'-phosphorothioate (Rp-cAMPS, 10 µM), an inhibitor of protein kinase A. In preparations pretreated with indomethacin (1 µM), CRF caused little change in the amplitude but significantly decreased the frequency of the contractions. Intracellular cAMP levels in the preparations, measured by enzyme immunoassay, rose when CRF was applied at 30 nM. These results suggest that the slight positive inotropic action of CRF is due to activation of both cAMP and prostaglandin pathways, whereas the negative chronotropic action is due solely to the cAMP signalling pathway.