In the rat, cytochrome P-450IA1 gene expression, which is most closely associated with aryl hydrocarbon hydroxylase activity, is thought to be regulated by several trans-acting factors, including the 4 S polycyclic aromatic hydrocarbon (PAH)-binding protein. This protein has been purified to homogeneity from rat liver using ion exchange, gel permeation, hydrophobic interaction, and affinity chromatographies. Partial sequencing of the 33-kDa band indicated its identity as glycine N-methyltransferase (GNMT). Polyclonal antibodies to GNMT immunoprecipitated PAH-binding activity from rat liver cytosol. Methyltransferase and PAH-binding activities copurified during the course of protein purification. GNMT protein and PAH binding activity were colocalized in various cytosolic fractions. These data all indicate that the 4 S PAH-binding protein and GNMT are one and the same protein or very similar proteins. Western blot analyses yielded a positive signal under denaturing (33 kDa) and nondenaturing (150 kDa, tetramer) conditions; the PAH-binding protein also was an oligomer. GNMT was detected by immunohistochemistry in nuclei from H4IIE rat hepatoma cells and rat liver. The localization of GNMT in liver nuclei is in accordance with a role in modulating cytochrome P-450IA1 gene expression.