1. Species differences in xenobiotic-mediated transcriptional activation of CYP3A genes are known to exist. These differences are proposed to be due, in part, to host cell differences. 2. Host cell effects were investigated by trans-species transient transfection of reporter genes containing either the rat CYP3A23 or human CYP3A4 proximal promoters into human HepG2 and rat FaO and H4IIEC3 hepatoma cells. HepG2 and FaO cells supported activation of both CYP3A constructs by xenobiotics in a species-specific manner, whereas H4IIEC3 cells were non-permissive. 3. The mRNA complement of the cell lines was then quantified by semiquantitative RT-PCR for adult CYP3As (CYP3A23, CYP3A4/5), steroid hormone receptors (constitutive androstane receptor, glucocorticoid receptor-alpha, pregnane X receptor) and transcription factors (Hepatic nuclear factor 4alpha, retinoid X receptor). 4. Principal component analysis of absolute receptor levels demonstrated a wide scattering, with no coherent pattern. In contrast, PCA of relative receptor ratios segregated H4IIEC3 cells from all other samples. 5. The observation is confirmed that species differences in response to xenobiotics are a result of host cell environment. In addition, new evidence is provided to support the hypothesis that in addition to individual receptor activation profiles, the relative abundance of steroid hormone receptors that control CYP3A gene expression play an important role in this observed species difference.