The hypotensive action of diltiazem by oral administration to conscious renal hypertensive dogs (one kidney, one figure 8) was studied and the effect was compared with that of nifedipine. Diltiazem decreased mean blood pressure 10-20 mmHg at doses of 1-4 mg/kg. The same doses of nifedipine exhibited hypotensive actions similar to diltiazem, but nifedipine induced a more pronounced reflex tachycardia than diltiazem. Combined administration of diltiazem with pindolol produced a greater hypotension than that caused by individual drugs and caused an increase in heart rate, smaller than by pindolol alone and larger than by diltiazem alone. When 60 mg of diltiazem was administered 3 times a day for 10 consecutive days, blood pressure decreased 15 mmHg on the third day or later. Although the time course of plasma level of diltiazem on the last day was similar to that on the first day, the heart rate initially increased slightly and decreased later. Prolongation of the PQ interval of an electrocardiogram was diminished after the fourth day. In conclusion, diltiazem decreased blood pressure of renal hypertensive dogs at doses comparable to those used for clinical treatment in acute and chronic experiments.