The extent of lipid peroxidation in vitro, as indicated by the production of malonaldehyde, was significantly different in homogenates of bovine and mouse intestinal mucosa. 2. Mouse intestinal mucosa was resistant to non-enzymatic lipid peroxidation whereas bovine intestinal mucosa was not. 3. Iron-dependent lipid peroxidation in bovine intestinal mucosa depends on the position the cells occupy along the crypt-villus axis. 4. The addition of methanolic extracts from bovine intestine to mouse liver homogenates produced a considerable increase in non-enzymatic peroxidation whereas those from mouse intestinal mucosa had no effect.