Nuclear receptors such as the constitutive androstane receptor (CAR) are central factors that link drug exposure to the activities of drug metabolism and elimination. In order to determine the in vivo effects of efavirenz, a CAR activator, the expression of target genes was determined in duodenal biopsies obtained from 12 healthy volunteers before treatment and after 10 days of treatment with efavirenz; concomitant administration of the cholesterol inhibitor ezetimibe produced no significant difference. However, in in vitro studies, efavirenz significantly increased CYP2B6 expression in several cell types, suggesting that the drug transactivates CAR. This hypothesis is supported by our findings that there is significant induction of CAR target genes in in vivo peripheral blood mononuclear cells (PBMCs) isolated from healthy volunteers treated with multiple doses of efavirenz. The impact of efavirenz on hepatic metabolism in vivo was confirmed by significant changes in plasma 4β-hydroxycholesterol and bilirubin levels and the area under the curve (AUC) of efavirenz. Induction of CYP2B6 mRNA expression correlated with the decrease in the AUC of efavirenz (r = 0.61; P = 0.036). Taken together, our results provide evidence that efavirenz exerts compartment-specific inductive capacity in vivo.