The NADPH- and oxygen-dependent metabolism of N,N-diethyl-m-toluamide (deet insect repellent) has been studied at 37 degrees C in suspensions of liver microsomes from phenobarbital-pretreated male Wistar rats. In pH 8 Tris-HCl buffer and at a substrate concentration of 200 microM, metabolic reactions corresponded to benzylic hydroxylation and N-deethylation, and to combinations of these reactions, leading to N,N-diethyl-m-hydroxymethylbenzamide, N-ethyl-m-toluamide, N-ethyl-m-hydroxymethylbenzamide, and m-toluamide. These compounds and N,N-diethyl-m-formylbenzamide were detected in methyl t-butyl ether extracts by capillary gas chromatography using three fused silica columns of different polarities. The structures of the in vitro metabolites were verified by comparisons with samples of authentic standards using combined gas chromatography-mass spectrometry. A selective derivatization procedure with acetic anhydride and pyridine facilitated the detection of the two alcohols. The major metabolites, N,N-diethyl-m-hydroxymethylbenzamide and N-ethyl-m-toluamide, were quantitatively determined in derivatized extracts with an internal standard of N,N-dipropyl-m-toluamide, a capillary column of DB-1, and a nitrogen-phosphorus detector. The mean enzymatic yield of these two products was 69% in three replicated experiments.