Dogs excreted approximately 45% of an oral dose of 14C-amitriptyline (30 mg/kg) in the urine in 24 hr. Two new urinary metabolites of the drug were identified as dihydrodiol derivatives of amitriptyline and nortriptyline, respectively. The major metabolite in dog urine was 10-hydroxyamitriptyline, excreted mainly in conjugated form. Other metabolites were characterized as 10-hydroxynortriptyline, amitriptyline N-oxide, and nortriptyline. Together, these metabolites accounted for approximately 47% of the urinary radioactivity.