Atrial natriuretic peptide exhibits natriuretic, diuretic and vasodilatory properties. We compared plasma concentrations of atrial natriuretic peptide, cyclic guanosine-3',5'-monophosphate (cGMP), electrolytes and urinary excretion of cGMP and electrolytes in hypertensive pregnant women to those in normotensive pregnant and normotensive non-pregnant women. Plasma atrial natriuretic peptide concentrations in hypertensive pregnant and normotensive non-pregnant women were equal, whereas in normotensive pregnant women it was lower (P < 0.05), than in non-pregnant. Urinary cGMP excretion was higher in both normotensive and hypertensive pregnant than in non-pregnant women (P < 0.01), whereas plasma cGMP levels were similar. A five-day nifedipine treatment (10 mg t.i.d.) had no effects on any of the variables. In hypertensive pregnancy, a reduction of systolic blood pressure by nifedipine correlated with the initial plasma atrial natriuretic peptide (P < 0.05) and a decrease in diastolic blood pressure with the initial plasma cGMP concentration (P < 0.05). The results of this small material suggest that plasma atrial natriuretic peptide concentration predicts the response to nifedipine in hypertensive pregnancy. However, the atrial natriuretic peptide-cGMP system does not seem to mediate the antihypertensive effect of nifedipine, while plasma atrial natriuretic peptide remained unaltered. Increased urinary cGMP excretion in both pregnant groups but lowered plasma atrial natriuretic peptide in normotensive pregnancy suggest other factors than circulating atrial natriuretic peptide to promote renal cGMP excretion during pregnancy.