The hydrolysis of trioctanoin and of monoolein by rat intestinal homogenates, catalyzed by the previously described "trioctanoin hydrolase" and "monoglyceride lipase" [Ref. 6 and 11], was shown to be entirely due to the action of the intestinal glycerol-ester hydrolase [Ref. 1 and 2] and to contaminating pancreatic lipase firmly bound to the intestinal cells. These conclusions are supported by experiments based on the chromatographic behaviour of the different lipolytic activities, the use of an immunoserum directed against rat pancreatic juice and the relief of bile salt inhibitions by pancreatic colipase. The results are in favour of the existence of a single lipolytic enzyme in the rat intestinal mucosa, i.e. glycerol-ester hydrolase, active against short- and medium chain mono-, di- and triglycerides and against long chain monoglycerides.