The disposition of moxalactam (MOX) and N-methyltetrazolethiol (NMTT) in rats and monkeys after intravenous injection was investigated, focusing on the in vivo liberation of NMTT, by using [NMTT-14C]MOX and [14C]NMTT. After [NMTT-14C]MOX injection, MOX levels in plasma quickly became high in both rats and monkeys and then declined, with half-lives at the beta phase of 18.8 and 67.1 min, respectively. The levels of NMTT liberated from MOX were much lower than those of MOX, but the apparent elimination was significantly slow. The levels of MOX and NMTT in rat liver were almost comparable but lower than those in plasma. With [14C]NMTT administration, the level of NMTT in plasma declined, with half-lives at the beta phase of 21.5 min in rats and 54.0 min in monkeys. After [NMTT-14C]MOX injection, most of the radioactivity was excreted in urine as MOX, with 11% of the dose in rats and 8% of the dose in monkeys eliminated as NMTT until 24 h. Total biliary excretion was 26% of the injected radioactivity in rats, and most of it was due to MOX. In one monkey, the total biliary excretion was only 0.2% of the injected radioactivity. With [14C]NMTT administration, most radioactivity was excreted in the urine as unchanged NMTT in both animals. Oral administration in rats showed that part of the biliary-excreted MOX was degraded to NMTT in the intestine and then absorbed. Repeated administration of [NMTT-14C]MOX to rats did not change the levels of MOX and NMTT in plasma or liver nor did it change the excretion profiles. Thus, accumulation of MOX and NMTT did not occur.