Adaptation of the maternal cardiovascular system to pregnancy is characterized by a reduction in vascular resistance and hypotension; by the third trimester, however, hypertension occurs in 5% of women. Nitric oxide (NO), synthesized from L-arginine by vascular endothelial cells, plays an important role in the control of systemic blood pressure. This study has assessed the relationship between NO synthesis and blood pressure in 12 normal pregnant and 13 hypertensive pregnant women. NO synthesis was determined as urinary nitrite (NO₂^-) and nitrate (NO₃^-) by chemiluminescence. Urinary NO₂^- excretion did not differ significantly between the hypertensive and normal women (median [range], 122 [68-324] and 147 [32-417] mmol/mmol creatinine, respectively; p0.05). After reduction of urinary NO₃^- to NO₂^-, urinary NO_x excretion (NO₃^- + NO₂^-) was also similar in the hypertensive and normotensive women (38 [0.3-112] and 28 [5-63] mmol/mmol creatinine, respectively, P>0.05). However, a significant correlation was seen in the hypertensive women between the change in systolic blood pressure and urinary NO_x excretion (σd² = 98.5, n = 13, P<0.01). These findings suggest that a compensatory increase in the synthesis of NO may occur in hypertensive pregnant women in an attempt to maintain homeostasis.