The hypotensive effect of acute and longterm, intravenous and oral administration of the calcium antagonist, diltiazem, was investigated in 8 normotensive volunteers and 55 patients with essential hypertension. Diltiazem i.v. infusion of 45 mg/h (0.5 mg/min, then 1.0 mg/min, each for 30 min rapidly decreased both blood pressure (BP) from 164 +/- 22/98 +/- 8 to 144 +/- 15/86 +/- 9 mmHg (mean +/- SD) and total peripheral resistance from 32.6 +/- 8.4 to 25.3 +/- 5.4 mmHg/l/min (p less than 0.001), and increased stroke volume from 58.2 +/- 9.5 to 64.2 +/- 8.6 ml/beat (p less than 0.05). It altered neither heart rate nor cardiac output in the hypertensives (n = 10). Oral diltiazem 60 mg rapidly decreased BP from 155 +/- 10/103 +/- 6 to 142 +/- 12/90 +/- 8 mmHg after 3 hours (p less than 0.01/p less than 0.001) in hypertensives (n = 8), but not in normotensives (n = 8). Diltiazem 90 mg p.o. decreased BP from 157 +/- 15/102 +/- 9 to 129 +/- 13/83 +/- 8 mmHg (p less than 0.01) in hypertensives (n = 15), and reduced the heart rate from 71 +/- 8 to 65 +/- 8 beats/min (p less than 0.01). The drug did not change plasma renin activity either in normotensives or hypertensives. The fall in diastolic BP was correlated with the plasma diltiazem concentration (r = 0.910, n = 6, p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)