Ten healthy individuals received in random order placebo, nifedipine 10 mg, nifedipine 20 mg, nitrendipine 10 mg and nitrendipine 20 mg as single oral administrations at weekly intervals. On the day before each treatment placebo was administered. Urine was collected for 6 h and then for 18 h after each administration. There was a significant increase in urine volume and sodium excretion after the drugs, but no change in potassium excretion. The effect was most evident in the 6 h after drug administration. The effect was no greater at the higher doses of either drug. A natriuretic-diuretic action of nifedipine and nitrendipine has been confirmed in man. The mechanism of the effect remains unclear.