The contents of [Met5]-enkephalin-Arg6-Gly7-Leu8 (met-enk-RGL) and its seven hydrolysis products-Y, YG, YGG, YGGF, YGGFM, YGGFMR, and YGGFMRG-were estimated after incubating met-enk-RGL with a membrane fraction from either guinea pig ileum or striatum for various times at 37 degrees C. After 15 min of incubation, met-enk-RGL was completely hydrolyzed in both the ileal and the striatal membrane preparations. The major hydrolysis products were YGGFMR, YGGF and Y, which indicates that dipeptidyl carboxypeptidase and aminopeptidase activities were mainly involved in the hydrolysis. Additionally, even when the ileal and the striatal preparations were incubated for 60 min in the presence of both captopril, a dipeptidyl carboxypeptidase inhibitor, and amastatin, an aminopeptidase inhibitor, 24% and 44% of enkephalin octapeptide, respectively, were hydrolyzed. The YGG fragment was the major hydrolysis product in both preparations. When the ileal and the striatal membrane fractions were incubated with met-enk-RGL in the presence of three peptidase inhibitors-captopril, amastatin, and phosphoramidon (an inhibitor of endopeptidase-24.11)-approximately 95% of the enkephalin octapeptide, remained intact in both cases. This shows that met-enk-RGL was almost exclusively hydrolyzed by three enzymes, amastatin-sensitive aminopeptidase, captopril-sensitive dipeptidyl carboxypeptidase I and phosphoramidon-sensitive endopeptidase-24.11, in both ileal and striatal membranes. We also reported the potencies of several opioids relative to that of met-enk-RGL in guinea pig ileum pretreated with the three peptidase inhibitors.