1,1'-Trimethylene [1,1'-14C]bis(4-aldoximinopyridinium) ion (TMB-4) was synthesized and its metabolic disposition was investigated in vivo. Rats were administered multiple doses of TMB-4 (25 mg/kg) by the intraperitoneal route and urine was collected over a 48-hr period. Approximately 98% of the administered radioactive dose could be accounted for in the urine during that time. A urinary metabolite, trimethylene-1-(4-aldoximinopyridinium)-1'-(4-carboxamidopyridinium) ion (TACARB), was isolated by ethanol extraction, charcoal adsorption chromatography, and ion-exchange chromatography. The metabolite was then characterized by comparing its spectral, chromatographic, and electrophoretic properties with those of authentic TACARB ion. Possible reaction mechanisms involved in the biochemical pathways for the formation of this metabolite from TMB-4 are discussed.