The aim of our study was to evaluate the effects of the calcium antagonist diltiazem (DTZ) on hemodynamics and regional blood flows measured with radioactive microspheres during the neonatal period. For this purpose, two bolus doses (0.5 and 1 mg/kg) of intravenous DTZ were injected in six 2- to 6-day-old urethan-anesthetized piglets. Six other piglets were injected with saline and served as controls. DTZ arterial plasma concentrations measured by high-pressure liquid chromatography were 0.521 +/- 0.034 and 1.141 +/- 0.073 microgram/ml, respectively, after 0.5 and 1 mg/kg. The only significant hemodynamic effects of DTZ were a decrease in mean arterial blood pressure and in heart rate. The main effect of DTZ on regional blood flow was a striking increase in coronary blood flow (by 54%) after both 0.5 and 1 mg/kg. To a lesser extent, DTZ increased significantly the blood flow to the brain, liver, ileum, diaphragm, and lungs. In contrast, DTZ at a dose of 1 mg/kg induced a significant decrease in renal blood flow (by 37%). We conclude that DTZ in the neonatal period is a nonspecific vasodilator. However, the decrease in renal blood flow induced by a dose of 1 mg/kg might be detrimental and requires further investigation.