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Objective: To evaluate the effects of angiotensin II and brain natriuretic peptide on the placental vasculature of diabetic women., Methods: Term placentas from five diabetic women and five nondiabetic controls were collected. Isolated placental cotyledons were perfused dually with fetal perfusion pressure as an index of vascular response. The effect of angiotensin II (10(-10)-10(-5) mol/L bolus injection) was established in the fetal-placental vasculature of all placentas in the absence or presence of brain natriuretic peptide (10(-8) mol/L final concentration). Data were analyzed using repeated measures analysis of variance and paired t test where appropriate., Results: A significant vasoconstrictor response to angiotensin II was achieved in placentas of both diabetic and nondiabetic women (P < .001); however, the angiotensin II-induced increase in perfusion pressure was significantly greater in the diabetic group (P < .01). Significant attenuation of vasoconstrictor response to angiotensin occurred in the presence of brain natriuretic peptide in placentas of both nondiabetic (P < .0025) and diabetic (P < .025) women, but the effect was more prominent in the diabetic group., Conclusion: The in vitro placental vasculature of diabetic women is more sensitive to angiotensin II than is the in vitro placental vasculature of nondiabetic women. The attenuation exerted by brain natriuretic peptide on angiotensin II-induced vasoconstriction is more prominent in placentas from diabetic women compared to those from nondiabetic women.