Nine metabolites of terodiline (N-tert-butyl-4,4-diphenyl-2-butylamine) have been identified in dog urine by various chromatographic techniques and mass spectrometry. The main metabolic pathway is aromatic hydroxylation, leading to the quantitatively most important metabolite, N-tert-butyl-4-(4-hydroxyphenyl)-4-phenyl-2-butylamine, and to two dihydroxylated metabolites, one mono substituted in both rings (N-tertbutyl-4,4'-bis(4-hydroxyphenyl)-2butylamine), and one disubstituted in one ring (N-tert-butyl-4-(3,4-dihydroxyphenyl)-4-phenyl-2-butylamine). The latter ' is further metabolized by methylation, forming N-tert-butyl-4-(4-hydroxy-3-methoxyphenyl)-4-phenyl-2butylamine, the second most abundant metabolite. Still another metabolite is formed by hydroxylation in the terr-butyl group to N-(2-hydroxymethyl-2-propyl)-4,4-diphenyl-2-butylamine. A very minor dihydroxylated metabolite results from oxidation both in an aromatic ring and in the tert-butyl group, giving N-(2-hydroxymethyl-2propyl)-4-(4-hydroxyphenyl)-4-phenyl-2-butylamine. Oxidation of the carbon adjacent to the nitrogen and subsequent deamination gives the two ketones 4-(4-hydroxyphenyl)-4-phenyl-2-butanone and 4-(4-hydroxy-3-methoxyphenyl)-4phenyl-2-butanone. Reduction of the carbonyl function in the former yields the corresponding alcohol, 444hydroxyphenyl)-4-phenyl-2-butanol. Some unchanged terodiline is also present. All metabolites formed by functionalization appear to be extensively conjugated, presumably with glucuronic acid.