To elucidate the nature of the iminoacidopathy in uremia, the prolidase and prolinase activities in erythrocytes from 36 normal subjects, 28 patients with chronic uremia, and 21 patients on maintenance hemodialysis were determined. A statistically significant decrease in the activities of both prolidase and prolinase was found in the uremic patients and the hemodialysis group. No difference has been observed in the enzyme activities in the pre- and postdialysis samples of the hemodialysis patients. It is suggested that decreased activities of prolidase and prolinase may contribute to the iminoacidopathy in uremia, including high contents of serum iminoacid containing peptides.