Rifampicin (rifampin) is a potent inducer of cytochrome P450 (CYP) 3A4. It was recently identified as a substrate of the polymorphic organic anion transporting polypeptide 1B1 (OATP1B1) expressed on the sinusoidal membrane of human hepatocytes. The present study aimed to investigate the possible association of single nucleotide polymorphisms (SNP) in the SLCO1B1 gene encoding for OATP1B1 with the inducing effect of rifampicin on hepatic CYP3A4. A total of 38 healthy volunteers who had participated in drug interaction studies with rifampicin were genotyped for the g. - 11187G > A and c.521T > C SNPs in SLCO1B1, c.3435C > T SNP in ABCB1 and g.6986A > G SNP in CYP3A5. The plasma concentration of 4beta-hydroxycholesterol, an endogenous marker of CYP3A4 activity, was measured before and after administration of 600 mg rifampicin once daily for 9-11 days. Treatment with rifampicin significantly increased the mean +/- SD plasma concentration of 4beta-hydroxycholesterol from 55.2 +/- 17.9 ng/ml to 120.9 +/- 32.0 ng/ml (P < 0.001). A large intersubject variability existed in the induction of CYP3A4 by rifampicin, but no associations were observed between the variability in induction and any of the polymorphisms studied. These data suggest that SLCO1B1 polymorphism does not affect the extent of induction of hepatic CYP3A4 by rifampicin, probably because other uptake transporters, such as OATP1B3, can compensate for reduced uptake of rifampicin by OATP1B1. However, the present study had sufficient power to detect only a considerably smaller rifampicin-induced increase in 4beta-hydroxycholesterol in carriers of the SLCO1B1 c.521C allele compared to subjects with the reference genotype.