Renal hemodynamics increase dramatically during pregnancy, and pressor responsiveness to exogenous administration of vasoconstrictors is attenuated. We investigated whether or not vasodilatory prostaglandins mediate these phenomena. Trained, chronically instrumented, conscious pregnant rats were used. Control values of glomerular filtration rate (GFR) and effective renal plasma flow (ERPF) were elevated at midgestation (P less than 0.01 and P = 0.05 from prepregnant means, respectively), and effective renal vascular resistance was decreased (P = 0.05). Indomethacin (4.5-6.5 mg/kg body weight [BW]) failed to decrease renal hemodynamics at this stage of pregnancy; in fact, it raised GFR somewhat further (P less than 0.05). Systemic pressor responsiveness to bolus administration of norepinephrine and angiotensin II (AII) was significantly attenuated by at least gestational day 20. Neither indomethacin (7 mg/kg BW) or meclofenamate (6 mg/kg BW) affected the refractory response. The renal vasculature was also relatively unresponsive to an intravenous infusion of AII (5 ng X kg-1 X min-1) during late gestation (day 19); in particular, the fall in ERPF in response to AII (16 +/- 3%) was markedly less than that observed in the prepregnant condition (34 +/- 3%; P less than 0.05). Indomethacin (6 mg/kg BW) failed to restore this blunted response, and further attenuation was evident, despite the presence of the inhibitor (gestational day 21). We conclude that vasodilatory prostaglandins do not appear to mediate the rise in renal hemodynamics, and the attenuation of the systemic and renal pressor responsiveness observed during pregnancy, insofar as these phenomena were unaffected by acute cyclooxygenase inhibition in unstressed, conscious rats.