We gave captopril (25 mg) to nine pregnant and eight non-pregnant women. From baseline to 60 min post-captopril, mean arterial pressure decreased significantly more in pregnant than in non-pregnant women (mean 15.0 [SE 2.5] vs 5.2 [1.2] mm Hg; difference 9.8 [95% CI 3.9-15.9] mm Hg, p = 0.0036), whereas plasma renin activity increased significantly more in the pregnant group (18.0 [5.5] vs 4.5 [1.9] ng mL-1 h-1; difference 13.5 [0.71-26.5], p = 0.043). These findings support the hypothesis that stimulation of the renin-angiotensin system in pregnancy maintains blood pressure.