Single-dose administration of 50 mg of 1-[2-[bis[4- (trifluoromethyl)phenyl]methoxy]ethyl]-1,2,5,6-tetrahydro-3- pyridinecarboxylic acid monohydrochloride resulted in temporary neurological and psychological symptoms in two subjects. Because of the nature of adverse effects, urine from a subject who received CI-966 orally was extracted to investigate the metabolism of CI-966 in man. An unknown urinary component was identified as a pyridinium metabolite of CI-966 based on HPLC-MS and 1H and 19F NMR. Structural confirmation was achieved by chromatographic and spectroscopic comparisons to a reference standard. In several in vitro screens and preclinical studies, the pyridinium metabolite appears to possess minimal pharmacological activity.