The acute effects of buccal nifedipine 20 mg on blood pressure, renal haemodynamics and electrolyte excretion were compared in 16 untreated patients (HT) with uncomplicated arterial hypertension (WHO I-II), 11 normotensives (NT) and 6 normotensives given a placebo. Nifedipine caused a significant fall in the systolic and diastolic blood pressures (BP) of 25.7 +/- 12/26.5 +/- 10 mmHg in the hypertensives, and a minor but significant fall in diastolic BP in the normotensives. Renal vascular resistance fell significantly and renal plasma flow was increased non-significantly in the hypertensives. No changes in these parameters were seen in NT. Glomerular filtration rate remained constant in all groups, also in HT despite the marked haemodynamic changes. Natriuresis was significantly increased to the same degree in the HT and NT groups, in spite of their different haemodynamic responses. Uric acid excretion showed a parallel acute increase in both groups. The significant and close relationship between the acute changes in the excretion of sodium and uric acid provides evidence for a proximal tubular natriuretic effect of nifedipine.