OBJECTIVE: This study was intended to test the hypothesis that glycosylated hemoglobin adversely affects endothelium-dependent vascular relaxation during pregnancy. STUDY DESIGN: Omental artery rings (3 mm) obtained from patients undergoing cesarean delivery at term were suspended in organ chambers for isometric tension recording in Krebs bicarbonate solution. They were bubbled with 5% carbon dioxide in air (37 degrees C, pH 7.4). After equilibration at 1-g passive tension and contraction with endothelin 1, concentration-relaxation curves in response to bradykinin were determined in the presence or absence of oxyhemoglobin, glycosylated hemoglobin, or nitric oxide synthase inhibitors (N(omega)-nitro-L-arginine methyl ester or N(omega)-nitro-L-arginine). RESULTS: Oxyhemoglobin and glycosylated hemoglobin attenuated the endothelium-dependent relaxation induced by bradykinin. The inhibition did not differ significantly between the 2 nitric oxide scavengers and was of the same magnitude as that noted with the nitric oxide synthase inhibitors. CONCLUSION: Glycosylation of hemoglobin does not change its effect on endothelium-dependent relaxation in human omental vessels during pregnancy. These data do not support the hypothesis that glycosylated hemoglobin may be the reason for abnormal endothelial function in diabetes.