Mechanisms that protect the body from a diverse array of harmful chemicals are also involved in drug metabolism, and can cause adverse drug-drug interactions. Two closely related orphan nuclear hormone receptors--the pregnane X receptor (PXR) and the constitutive androstane receptor (CAR)--have recently emerged as transcriptional regulators of cytochrome P450 expression that couple xenobiotic exposure to oxidative metabolism. In this review, we provide an examination of the roles of PXR and CAR as xenobiotic sensors, and discuss the application of this knowledge to toxicological screening in drug discovery.