The acute effects of a single dose of nifedipine (NIF, 20 mg SL) on heart rate (HR), arterial pressure (AP), forearm blood flow (FBF) and forearm vascular resistance (FVR), were measured in 15 normotensive (NP) and 11 hypertensive (HP) patients. In NP, NIF increased HR (from 78 +/- 3 to 88 +/- 3 lat/min, P less than 0.05) and FBF (from 4.0 +/- 0.4 to 5.5 +/- 0.3 ml/min. 100 ml, P less than 0.05) and decreased AP (from 99 +/- 2 to 89 +/- 3 mmHg, P less than 0.05) and FVR (from 29 +/- 3 to 22 +/- 2.9 relative units, P less than 0.05). In HP the changes were (with the exception of HR, which did not vary significantly) much greater in magnitude, and directly correlated with pretreatment levels of AP. In vitro experiments in aortic rings from spontaneously hypertensive (SHR) and normotensive (NR) rats demonstrated a much greater sensitivity of SHR to the relaxant effect of NIF, while the relaxant effect of nitroglycerin was similar in NR and SHR. It is concluded that NIF, although acting preferentially in HP as a hypotensive drug, also exerts a slight but still significant effect in NP. The greater vasodilating effect in HP could be due to a supersensitivity of the smooth muscle cell to NIF, possibly located at the calcium influx level.