We administered diltiazem HCl i.v. (0.05 mg/kg in bolus followed by 0.01 mg/kg/min for 45 min), and determined the changes in blood pressure (BP), glomerular filtration rate (GFR), renal blood flow (RBF), total renal resistance, urinary volume (UV), urinary sodium (UNa) and potassium excretion, urea and osmolar clearance, and tubular reabsorption ratio of sodium (TRNa%). The serum concentration of diltiazem achieved was similar to the maximum level after a single oral dose of 120 mg. GFR and RBF were measured by i.v. infusion of sodium thiosulfate and sodium rho-amino-hippurate, respectively, as indicators. The subjects included 12 cases of essential hypertension (EH), 10 of chronic glomerular nephritis (CGN) with hypertension, 12 of CGN without hypertension, 12 of ischemic heart disease (IHD), and 10 of normotensive controls. BP decreased in hypertensives but not in normotensives. In patients with EH, GFR and RBF increased markedly (by 25.3 +/- 33.8% and 30.7 +/- 39.5%, respectively). In patients with IHD, GFR increased slightly by 9.8 +/- 17.6%, whereas in patients with CGN with hypertension, GFR decreased by -4.3 +/- 14.3%. No significant change of these indices was observed in normal subjects and in patients with CGN without hypertension. UV and UNa increased and TRNa% decreased in all groups. Urea and osmolar clearance increased in almost every group.