The effect of durations of diltiazem pretreatment on nifedipine kinetics was evaluated. Eighteen healthy male subjects were randomly allocated to three groups, 6 subjects each, for single doses of 60 mg diltiazem, and 3 days and 6 days with diltiazem 60 mg three times a day. All subjects received 20 mg nifedipine orally on two occasions using a double-blind cross-over, placebo-controlled method. No significant difference on pharmacokinetic parameters of nifedipine without diltiazem were observed among three groups. The single dose with 60 mg diltiazem significantly increased the area under the plasma concentration-time curve (AUC) for nifedipine compared with that in control an average of 35.1% (P < .05) and decreased the total body clearance (CL) an average of 24.0% (P < .05). Three days and 6 days pretreatment with diltiazem 60 mg three times a day significantly increased mean nifedipine AUC to 151.1% (P < .01), 188.0% (P < .05) of control values, and decreased CL to 58.2% (P < .01), 63.9% (P < .05) of control values, respectively. The elimination half-life (t1/2) of nifedipine were significantly prolonged both after 3 days' and after 6 days' pretreatment of diltiazem. These results suggest that diltiazem affects the nifedipine kinetics rapidly and pretreatment duration dependently. A clinically important drug interaction may occur when both drugs are administered simultaneously.