[en] During the discovery of enkephalins by Hughes it was observed that these peptides were susceptible to rapid enzymatic inactivation. Since that time considerable work has been done to characterize the route of catabolism of [Met]enkephalin (ME) in brain and selected peripheral tissues. However, little has been done to characterize the metabolic disposition of the enkephalins in the peripheral circulation. The purpose of this study was to identify the mode of deactivation of ME in rabbit plasma. In vitro plasma half life (t1/2) of ³H-ME was determined following incubation of freshly isolated rabbit plasma at 37⁰C. In vivo 1/2 was determined following a bolus injection of ³H-ME. ³H-ME was extracted from plasma by chromatographic separation using Amberlite XAD-2 columns. Degradation was temperature dependent with a t1/2 of 1.5 +/- 0.3 min at 37⁰C. In vitro catabolism was dose dependently inhibited in the presence of either bacitracin, bestatin, puromycin or thiorpan. The t1/2 in the presence of 20 μM of each inhibitor was 2.4 +/- 0.3, 7.1 +/- 0.6, 2.4 +/- 0.3 and 3.2 +/- 0.2 min, respectively. The in vivo t1/2 was 0.5 +/- 0.1 min. Pretreatment of rabbits with a combination of 50 mg/kg bacitracin, 1 mg/kg thiorphan and 1.5 mg/kg puromycin produced a 1.6 fold increase in the in vivo t1/2 (0.8 +/- 0.1 min). Thus, circulating levels of ME are rapidly inactivated primarily by an aminopeptidase