The fate of protein conjugate of desacetyl-alacepril (DU-1227) and its effect on angiotensin I converting enzyme (ACE) activity in renal hypertensive rats were studied. [14C]DU-1227-protein conjugate was prepared by ultrafiltration method and administered intravenously in rats. Elimination of radioactivity of [14C]DU-1227-protein from plasma after injection seemed much slower than that reported of [14C]alacepril (1-[(S)-3-acetylthio-2-methylpropanoyl]-L-prolyl-L-phenylalanine, DU-1219) given orally. In the plasma unbound fraction, captopril and captopril-cysteine were detected. Most tissue levels were higher than plasma levels. Significant reduction of tissue ACE activity was seen after administration of the conjugate. Radioactivity was mostly excreted in feces. Captopril, captopril disulfide and captopril-cysteine were found as urinary metabolites. These findings indicate that protein-bound DU-1227 readily dissociated and released DU-1227 was converted to captopril in vivo and can therefore participate in prolonged hypotensive effect exerted by alacepril.