The role of endothelium in the decreased contraction to phenylephrine in aorta of pregnant Wistar rats has been studied. Contractions in response to phenylephrine were significantly decreased in aortic rings from pregnant rats compared to those from non-pregnant rats. Pretreatment of the rings with indomethacin did not significantly alter the pattern of responses. When endothelium was removed from the rings, contractions of rings from non-pregnant rats were similar to those from pregnant rats. Acetylcholine-induced relaxations of rings from pregnant and non-pregnant rats precontracted with phenylephrine were not significantly different. The results suggest that in the rat aorta and under in vitro conditions, pregnancy-induced attenuation of contraction to phenylephrine is, at least partly, endothelium dependent and that the release of the endothelium factor responsible for this alteration in response may be indomethacin insensitive.