Recently, a new type of interaction has been reported in which fruit juices diminish oral drug bioavailability through inhibition of organic anion-transporting polypeptide (OATP). In this study, we aimed to clarify the duration of OATP inhibition by grapefruit juice (GFJ), and to compare it with the duration of GFJ-induced inhibition of cytochrome P450 (CYP) 3A4 activity. Seven healthy volunteers were enrolled in this open-label, single-sequence study. They were orally administered celiprolol (100 mg) and midazolam (15 µg/kg) with water on the control day. Three days later, they ingested GFJ (200 mL) 3 times a day for 3 d. On day 1, the same drugs were administered with GFJ. On days 3 and 7, the same drugs were administered with water. Pharmacokinetics of both drugs were evaluated on each trial day. The peak plasma concentration ( C _max) and the area under the plasma concentration–time curve from 0 to 8 h ( AUC _0–8) of celiprolol significantly decreased on day 1, and the mean ratios of these values and the corresponding control-day values were 0.18 and 0.25, respectively. The C _max and AUC _0–8 returned to the control levels on days 3 and 7. In contrast, AUC _0–8 of midazolam were higher on days 1 and 3 than on the control day (mean ratio, 2.12 and 1.47, respectively). The AUC _0–8 returned to the control level on day 7. In conclusion, results of this study indicated that the OATP inhibition caused by GFJ dissipated faster than GFJ-mediated alterations in CYP3A4 activity, which were sustained for at least 48 h.