1. Biotransformation and excretion of xilobam (Xm) were studied after single oral doses of Xm-14C in mouse, rat, dog and man. 2. Following oral administration of Xm-14C, recoveries of total 14C (0-24 h) in urine were > or = 78% of the dose in all species. 3. Xm and a total of 11 metabolites have been isolated and identified, which accounted for 30, 65, 21 and 49% of the total 14C in the urine samples from mouse, rat, dog and man, respectively. 4. Xm was sequentially oxidized at the pyrrolidine ring to form 5'-OH Xm and 5'-oxo Xm. Both metabolites were isolated from human plasma accounting for 61% of the radioactivity in the sample. 5'-OH Xm was also identified as a major in vitro metabolite in the 9000g supernatant from a rat liver homogenate preparation. 5. 5'-OH Xm was isolated from the urine of all species except rats. However, oxidation products of 5'-oxo Xm were also present. Oxidation at the phenyl (ph) ring and at the phCH3 group produced the corresponding 4-OHph and phCH2OH metabolites. Subsequent water addition at the 2-position of the pyrrolidine ring followed by cleavage and/or cyclization of the above metabolites resulted in six additional urinary metabolites.