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<.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 posthydralazine 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.