BACKGROUND The possible influence of the variations in blood pressure and the plasma renin activity (PRA) after the administration of nifedipine (NIF) on the natriuretic effect of this calcium antagonist were evaluated. METHODS The differences in the values of sodium excretion and tubular reabsorption were evaluated in 18 patients with essential hypertension with the method of the lithium clearance before and after the administration of a sublingual NIF dose. RESULTS An increase in sodium excretion at the expense of a smaller distal reabsorption was found after NIF administration, without differences in patients with (n = 9) or without (n = 9) increase in PRA after NIF administration. The differences in several parameters when patients were classified depending on whether their mean blood pressure was reduced (n = 8) in more than 10% or not (n = 10) 90 minutes after NIF administration are discussed. CONCLUSIONS Natriuresis induced by nifedipine is due to a diminished distal reabsorption of sodium. This effect is independent of PRA or its changes. On the other hand, the differences found in subgroups with different blood pressure response support the hypothesis that there are two populations of patients with essential hypertension depending on their acute response to calcium antagonists.