The present study was undertaken to determine if three sources of dietary fiber would alter length, weight, DNA, protein or enzyme activity in the small intestine since various fibers are known to decrease intestinal absorption. Rats were fed semipurified diets that contained either 20% cellulose (C), 20% oat bran, 5% pectin (P) or no fiber source (FF). Leucine aminopeptidase activity were significantly greater in the P and C groups when compared with the FF group. There were no significant differences in sucrase activity. Animals in the P group had heavier and longer small intestine and heavier mucosa than the FF group. There were no significant differences in total mucosal DNA or protein. These results indicate that two sources of dietary fiber, cellulose and pectin, can change leucine aminopeptidase activity in the small intestine.