To investigate how the effects of oxytocin on blood pressure are influenced by female sex hormones, oxytocin (1 mg/kg, .) was given to intact cycling and ovariectomized (OVX) female rats. Oxytocin caused a transient increase in blood pressure, most pronounced during proestrus (p < ) and estrus (p < ). This increase was partially antagonized by an oxytocin antagonist. When oxytocin was given for 5 days, blood pressure decreased (intact rats: 123+/- vs. 130+/- mm Hg; p < , OVX rats: 120+/- vs. 129+/- mm Hg; p < ). This decrease, not abolished by the oxytocin antagonist, persisted for 3 weeks in intact rats and for 8 days in OVX rats. If oxytocin treatment of OVX rats continued, a nadir of 12 mm Hg (118+/- mm Hg; p < ) was reached after 8 days. Thereafter heart rate decreased significantly (p < ). One daily oxytocin injection for 12 days to OVX rats decreased blood pressure for 3 weeks, as in intact rats. These results show that acute and chronic oxytocin treatment cause opposite effects on blood pressure, and that these effects are modified by female sex hormones.