Following a two-week placebo period a group of 10 elderly men (63-76 years) received a 20 mg nifedipine tablet each. Twelve hours later nifedipine, as compared to placebo, significantly raised diuresis, urine sodium excretion, and tubular sodium rejection, but had no effect on GFR and urine potassium excretion. A significant decrease in mean arterial BP was registered in the 6th hour following nifedipine. In spite of persisting natriuresis, the statistical significance of the antihypertensive effect dropped tenfold in the 12th hour; moreover, correlation analysis did not show any relationship between natriuresis and BP drop. Although the mechanism of the natriuretic effect of Ca2+ channel blockers is not yet fully understood, it seems that the increase in Na+ excretion might be due to inhibition of Na+ reabsorption in renal tubules. It appears, though, that, following a single dose of nifedipine, the changes in Na+ excretion did not immediately contribute to the BP decrease.