The hemodynamic changes after intravenous administration of hydralazine were assessed using M-mode echocardiography in 13 pregnant patients with severe hypertension. The patients were divided into two groups: eight who had preeclampsia (group 1) and five who had essential hypertension and/or superimposed preeclampsia (group 2). Patients in group 1 had significantly lower pretreatment cardiac indexes and higher systemic vascular resistances (P less than .01) than did patients of group 2, despite similar mean blood pressure values. Thirty minutes after intravenous administration of 12.5 mg hydralazine, a significant decrease in mean arterial pressure (21%) and systemic vascular resistance (41%) and also a significant increase in heart rate (22%) and cardiac index (33%) were observed in group 1. The hemodynamic response in group 2 was attenuated; the mean arterial pressure and systemic vascular resistance fell by an average of 11 and 13%, respectively. The post-hydralazine percent changes in mean blood pressure for the total patient population did not correlate with the initial values of blood pressure (r = .091). In contrast, the percent reduction in systemic vascular resistance after administration of hydralazine inversely correlated with the control values of vascular resistance (r = -.742). These findings indicate that cardiovascular response to acute hydralazine administration is related to the baseline hemodynamic setting. M-mode echocardiography seems to be a reliable method for comprehensive analysis of cardiovascular effects of pharmacologic intervention in pregnant women with hypertension.