Background/Aim The objective was to evaluate the effect of single and repeated grapefruit juice relative to water on oral pharmacokinetics (PK) of the non-metabolized and P-glycoprotein (Pgp)-transported drug talinolol in humans, and to assess the impact of grapefruit juice on Pgp and intestinal uptake transporters. Methods Oral PK of 50mg talinolol was determined with water, single (300mL), and repeated grapefruit juice intake (6 days, 900mL/day) in 24 healthy Caucasians. MDR1 mRNA and Pgp levels were measured in duodenal biopsies of 3 subjects before and after juice. All subjects were genotyped for three MDR1 polymorphisms (1236C>T, 2677G>T/A, 3435C>T). Results Single grapefruit juice decreased the talinolol AUC, Cmax, and urinary excretion values to 56% (P<.001), 57% (P<.001) and 58% (P<.001), respectively, of those with water; repeated grapefruit juice showed a 50 to 65% reduction (P<.01). Single or repeated juice intake did not affect CLR, t1/2, and tmax. MDR1 mRNA and Pgp levels in duodenal biopsies did not differ. MDR1 genotypes were not associated with altered PK of talinolol. Conclusion Since both single and repeated juice intake lowered, rather than increased talinolol AUC without changing Pgp expression, our findings suggest constituents in grapefruit juice preferentially inhibited intestinal uptake rather than Pgp. Clinical Pharmacology & Therapeutics (2005) 77, P4–P4; doi: 10.1016/j.clpt.2004.11.019