1. Lactase, sucrase, maltase, trehalase and alkaline phosphatase activities of rat proximal jejunum were measured in 3, 6, 9, 12, 18 and 24-month-old rats fed with diets differing in their fatty acid composition. 2. A drop of 47-53% of the specific enzyme activity was observed with disaccharidases against a decrease of 71% for alkaline phosphatase in the 24-month-old rats compared to the 3-month-old rats. 3. Changes in dietary fatty acid composition, either in the saturated or monounsaturated ratio, or in the polyunsaturated fatty acid composition, did not significantly interfere with this aging effect.