The results of quantitative structure-activity relationships (QSARs) are reported for several series of cytochrome P450 inducers, including those which also act as ligands for the various nuclear receptors involved in regulation of the relevant P450 genes, namely, CYP1, CYP2, CYP3 and CYP4. In several examples presented, the QSARs are consistent with homology modelling studies of the nuclear receptor ligand-binding domains (LBDs) based on available crystal structures of the oestrogen and peroxisome proliferator-activated receptors' LBDs. Good correlations (R=0.91-0.99) are found between various structural parameters and biological activity (either in the form of P450 induction or ligand-binding affinity) for the Ah receptor (AhR), human estrogen receptor alpha (hER alpha), human glucocorticoid receptor (hGR) and the rat peroxisome proliferator-activated receptor alpha (rPPAR alpha).