Plasma concentrations of noradrenaline and adrenaline were measured serially during normal pregnancy in 52 women who remained normotensive and in nine who developed pregnancy-induced hypertension but were otherwise normal. There was a significant fall in both noradrenaline (P less than 0.005) and adrenaline (P less than 0.05) levels during normal pregnancy. Throughout pregnancy the mean levels of both catecholamines were lower in the patients who developed pregnancy-induced hypertension than in the normotensive patients. There was no evidence of increased sympathetic nervous system activity in the women who developed pregnancy-induced hypertension.