Plasma renin activity (PRA) and plasma aldosterone concentrations were determined in thirty-four normotensive pregnant subjects, sixteen subjects who were examined 6 weeks post-partum and sixteen non-pregnant controls. Plasma renin activity and plasma aldosterone concentrations increased sequentially in pregnancy i.e. there were progressive significant increases when the values of the first, second and third trimesters were compared. The values for the post-partum subjects and the non-pregnant controls were not significantly different. There was no significant correlation between any pair of the following indices: mean arterial pressure (MAP), PRA and plasma aldosterone. The results are discussed in the light of the present state of knowledge of the pathogenesis of pregnancy-induced hypertension (PIH).