E-64 isolated from a culture of Aspergillus japonicus is a specific inhibitor of cysteine proteinases. E-64-c, a synthetic analog of E-64, was effective in model animals of muscular dystrophy only when it was given intraperitoneally and by means of osmotic minipump. It showed no effects due to its low absorbability from intestine when it was administered orally. EST, the ethyl ester of E-64-c, was expected to be readily absorbed through intestinal membrane, since it is more lipophilic than E-64-c. Both EST and E-64-c have a high specificity to cysteine proteinase similar to E-64 but E-64-c was 100 to 1000 times stronger than EST in in vitro cathepsin inhibition. However, EST was stronger than E-64-c in cathepsin inhibition when given orally. The cathepsin B&L activities (whole activities of cathepsins B and L) in the skeletal muscle, heart and liver of hamsters were strongly inhibited soon after oral administration of 100 mg/kg body weight of EST. The inhibition continued for at least 3 h and then disappeared gradually. E-64-c was found in plasma of hamster treated with EST, but unchanged EST was not found. These results suggested that EST was converted to E-64-c, a more active form, during the permeation through intestinal membrane. The conversion of EST to E-64-c was also indicated by the absorption experiment using in situ loop method. EST was thus shown to be useful as an oral drug and expected to be effective in therapeutic trials using model animals.