Pyridine has been shown to induce hepatic microsomal cytochrome P450IIE1 and dependent activities. In the present study a single dose of pyridine (200 mg/kg body weight i.p.) induced cytochrome P450IIE1 in both the liver and lung. Increases were observed in p-nitrophenol hydroxylation, ethoxyresorufin deethylation and N-nitrosodimethylamine metabolism in both tissues. Pyridine appears to be an excellent inducer of this isozyme in the lung and may be useful in studying xenobiotic metabolizing activities in this organ which are dependent upon this isozyme.