Adult female Sprague-Dawley rats were fed isocaloric semipurified diets containing a high content of either polyunsaturated (P) or saturated (S) fatty acids; these diets were nutritionally adequate, providing for all known essential nutrient requirements. On day 3 after beginning S or P, one group of animals was exposed to a single 6-Gy dose of abdominal radiation, and the other half was sham irradiated. S or P diets were continued for a further 14 days. Brush-border membrane purification and sucrase-specific activities were unaffected by diet or by abdominal irradiation. In rats fed P, irradiation was associated with an increase in jejunal brush-border membrane total phospholipid and the ratio of phospholipid to cholesterol; these changes were not observed in animals fed S. In irradiated rats, ileal brush-border membrane phospholipid per cholesterol was high in animals fed S compared with P. In irradiated animals fed P, there was reduced jejunal and ileal uptake of several medium- and long-chain saturated and unsaturated fatty acids and cholesterol, and the ileal uptake of higher concentrations of glucose was reduced in irradiated animals fed P. In contrast, lipid uptake was similar in control and irradiated animals fed S except for cholesterol uptake, which was reduced. Ileal uptake of higher concentrations of glucose was increased in irradiated animals fed S. Quantitative autoradiography failed to demonstrate any change in the distribution of leucine or lysine transport sites along the villus 1 or 2 wk after abdominal irradiation or in response to feeding S or P. Also, these differences in transport achieved by feeding S to radiated animals were not explained by variations in the animals' food consumption or intestinal mucosal surface area. Thus the use of short-term feeding with a saturated fatty acid diet in the prevention of acute irradiation damage to the intestine warrants further investigation in humans.