The main metabolites of diltiazem in rats are acidic metabolites having a carboxyl group which may be formed by oxidative deamination of the dimethylaminoethyl group of diltiazem. In order to identify the enzymes responsible for the deamination and formation of acidic and neutral metabolites [14C]diltiazem was incubated with microsomal and mitochondrial preparations from the liver of SD male rats. Both acidic and neutral metabolites were formed only in the presence of an nicotinamide adenine dinucleotide phosphate generating system. Their formation was remarkable, especially in the microsomes, and inhibited by SKF 252-A, but not by pargyline and iproniazid. The production of neutral metabolites surpassed that of acidic ones. Structural analysis by gas chromatography-mass spectrometry showed that the neutral metabolites are aldehydes which have not been detected in vivo. The results suggest that the dimethylaminoethyl group of diltiazem is oxidized to an aldehyde group by microsomal cytochrome P-450 in the liver. Subsequently, the aldehyde group would be dehydrogenated to the carboxyl group.