Transcription of genes encoding cytochrome P450 3A (CYP3A) monooxygenases is induced by a variety of xenobiotics and natural steroids. There are marked differences in the compounds that induce CYP3A gene expression between species. Recently, the mouse and human pregnane X receptor (PXR) were shown to be activated by compounds that induce CYP3A expression. However, most studies of CYP3A regulation have been performed using rabbit and rat hepatocytes. Here, we report the cloning and characterization of PXR from these two species. PXR is remarkably divergent between species, with the rabbit, rat, and human receptors sharing only approximately 80% amino acid identity in their ligand-binding domains. This sequence divergence is reflected by marked pharmacological differences in PXR activation profiles. For example, the macrolide antibiotic rifampicin, the antidiabetic drug troglitazone, and the hypocholesterolemic drug SR12813 are efficacious activators of the human and rabbit PXR but have little activity on the rat and mouse PXR. Conversely, pregnane 16α-carbonitrile is a more potent activator of the rat and mouse PXR than the human and rabbit receptor. The activities of xenobiotics in PXR activation assays correlate well with their ability to induce CYP3A expression in primary hepatocytes. Through the use of a novel scintillation proximity binding assay, we demonstrate that many of the compounds that induce CYP3A expression bind directly to human PXR. These data establish PXR as a promiscuous xenobiotic receptor that has diverged during evolution.