1. In various preparations of rat small intestinal epithelial cells the acylation and deacylation of monoacylglycerol were studied. In the in vitro vascularly perfused intestine, of which the lumen was loaded with monoacylglycerol with or without fatty acids, acylation exceeded deacylation. In contrast, deacylation was much faster in isolated microsomes and in isolated whole cells. 2. In vascularly perfused intestine, without long-chain fatty acids present in the lumen, the amount of di- and triacylglycerol formed was found to be half of that formed in perfusions with long-chain fatty acids in the lumen, while the glycerol formation was increased 1.4-fold. 3. The concentration of monoacylglycerol is an important factor in determining the relative rates of monoacylglycerol acylation and deacylation in microsomes: the ratio acylation/deacylation decreases with increasing monoacylglycerol concentrations. 4. The function of the monoacylglycerol lipase in fat resorption is discussed.