A clinical study was performed in eight healthy volunteers to investigate the effect of various timing of grapefruit juice intake on nisoldipine pharmacokinetics and pharmacodynamics, and to validate our pharmacokinetic model. The subjects were given 10 mg oral nisoldipine with water (control), or 5 mg oral nisoldipine with 200 mL grapefruit juice (G0) or with water at 14 (G14), 38 (G38), 72 (G72) or 96 hours (G96) after a 7-day period of thrice-daily intake of grapefruit juice. Grapefruit juice ingestion did not affect heart rate or the effect area during the first 8 hours of heart rate after nisoldipine administration, although significant decreases of systolic and diastolic blood pressure were caused in G0 by coadministration of grapefruit juice with nisoldipine. Headaches were reported by 3, 2, and 1 persons in G0, G14, and G38, respectively, but no subjects in G72 and G96 reported headaches. Compared with the control group, the maximum plasma concentration of nisoldipine was significantly increased after grapefruit juice intake in G0 and G14, and the plasma concentration was significantly increased at each time in G0 to G72. Therefore the effect of grapefruit juice decreased time dependently and lasted for at least 3 days after intake. Furthermore, our model gave predicted values in good agreement with the observed values. It is therefore necessary to withhold grapefruit juice for at least 3 days before administration of the drug to prevent grapefruit juice-nisoldipine interaction.