—— The purpose of this study was to examine the role of the renin-angiotensin system in mediating the hypertension in response to chronic reductions in uterine perfusion pressure (RUPP) in conscious chronically instrumented pregnant rats. Mean arterial pressure was significantly higher in pregnant rats with chronic RUPP (125±3.0 mm Hg, P<0.01, n=12) than in pregnant rats (100±2.3 mm Hg, n=17). Plasma renin activity in pregnant rats with chronic RUPP was 17.1±2.5 nmol angiotensin I · L⁻¹ · h⁻¹ compared with 21.9±3.5 nmol angiotensin I · L⁻¹ · h⁻¹ in pregnant rats. Chronic oral administration of a converting-enzyme inhibitor (enalapril, 250 mg/L for 6 days) decreased mean arterial pressure to a similar extent in pregnant rats with chronic RUPP (109±4.2 mm Hg, P<0.01, n=9) and in normal pregnant (81±1.8 mm Hg, P<0.01, n=9) rats. Blockade of the renin-angiotensin system, however, had no significant effect on the blood pressure response to chronic RUPP as differences were similar in control (Δ25 mm Hg) and converting-enzyme inhibitor-treated (Δ27 mm Hg) groups. These findings suggest that the renin-angiotensin system does not play a major role in mediating the hypertension produced by chronic RUPP in pregnant rats.