We have hypothesized that an alteration in the production of endothelium-dependent factors by sex hormones is a potential unifying mechanism for both the decreased arterial contractility and the redistribution of cardiac output characteristic of normal pregnancy. Thus, the effect of pregnancy/ estradiol on any one vascular bed will reflect the number and distribution of estrogen receptors. In this article, we review what is known about the effects of pregnancy and estrogen on nitric oxide synthase. Pregnancy increases Ca(2+)-dependent NOS activity early in gestation. The timing of the increase parallels the increase in plasma estradiol concentration. The increase in maternal brain NOS during pregnancy is blocked by tamoxifen. cGMP content increases along a similar time course in most but not all tissues. The changes in cGMP more closely approximate the changes in blood flow during pregnancy. This suggests that multiple elements of the NO:cGMP pathway are altered by pregnancy. It also shows that cGMP content cannot always be used as a surrogate for NOS activity. Estradiol, but not progesterone or testosterone, increases CA(2+)-dependent NOS activity. NO accounts for some, but not all of the pregnancy-associated changes in maternal arterial contractile response. It is not involved in uterine quiescence. Nitric oxide synthase is developmentally regulated in the fetus and is likely important in regulating the distribution of fetal blood flow.