1. Nitric oxide released from endothelial cells is a potent vasodilator that might play an important role in cardiovascular regulation during pregnancy. Platelets, like endothelial cells, contain a constitutive form of nitric oxide synthase. 2. The present study aimed to measure the activity of this nitric oxide-forming enzyme in normotensive pregnant and non-pregnant women, as well as in women who had developed pre-eclampsia. Nitric oxide synthase activity was measured in the platelets of 21 normotensive pregnant women, 16 non-pregnant women and seven pregnant women who had developed pre-eclampsia. 3. The nitric oxide synthase activity was significantly higher in normotensive pregnant women [36.8 +/- 2.7 pmol h-1 mg-1 of protein (mean +/- SEM), P < 0.001] than in non-pregnant control subjects (16.8 +/- 1.4 pmol h-1 mg-1 of protein) and in women with pre-eclampsia (24.5 +/- 2.1 pmol h-1 mg-1 of protein, P < 0.01). 4. These data suggest that nitric oxide synthesis is increased during normal pregnancy, possibly contributing to the vasodilatation associated with this condition. Nitric oxide generation, however, may be inappropriately low in pregnant women developing pre-eclampsia, thus leading to an enhanced vasoconstriction.