The present study was designed to (1) outline, in a prospective manner, vascular responsiveness to angiotensin II (A-II) in gravidas with essential hypertension and (2) ascertain if increased sensitivity to infused A-II could be detected before worsening of hypertension developed in those women who were destined to develop superimposed pregnancy-induced hypertension (PIH). Sixty-three women with chronic essential hypertension were studied sequentially throughout pregnancy and two groups of patients were defined on the basis of clinical outcome and serial measurements of vascular reactivity to exogeneously administered A-II. The first group consisted of 29 gravidas with chronic hypertension alone and the second group was composed of 34 patients with chronic hypertension who were destined to develop superimposed PIH. Vascular resistance to infused A-II (more than 7 ng. per kilograms per minute required to increase diastolic blood pressure 20 mm. Hg) was observed from weeks 21 to 27 of gestation in both groups of patients. In the gravidas with essential hypertension alone, resistance to A-II persisted throughout the remainder of pregnancy. However, in those women destined to develop PIH, an increased sensitivity to A-II developed after week 27 of gestation and the differences in mean values between the two groups of patients became highly significant after week 30 of gestation (P less than 0.025). although individual determinations obtained in both groups of patients between weeks 28 and 32 of preganncy suggested that pressor responsiveness to A-II might be used as a screening technique to identify those women destined to develop PIH, additional studies conducted between weeks 28 and 32 of gestation will be required before the efficacy of such a screening technique is clearly established.