An endophytic fungus found in Kentucky 31 (KY-31) tall fescue has been associated with the "summer syndrome" (fescue toxicity) in grazing cattle. Four experiments were conducted to investigate the use of weanling rats as an animal model to study the fescue toxicity syndrome. When graded levels of fungus-infected (INF) or fungus-free (FF) KY-31 seed were fed, feed intake and growth rates were inversely related to the amount of INF seed in the diet. When diets containing 50% seed were fed, rats eating INF seed had reduced (P less than .05) daily water consumption (9 vs 20 ml) and lowered (P less than .05) body temperatures (36.8 vs 37.7 C) when compared with the FF treatments. Autoclaving the seed increased feed intake and growth rate of rats receiving both INF and FF seed diets, but daily gains continued to be less (P less than .05) in the rats fed autoclaved INF diets when compared with rats fed autoclaved FF diets (2.3 vs 6.8 g). Rats fed diets containing either 40% shredded wheat on which the fescue endophyte had been grown or 40% shredded wheat that was not innoculated did not differ in performance. Crude fiber, nitrogen-free extract, gross energy and dry matter digestibilities were enhanced (P less than .05) by the presence of INF seed in the diets when compared with the FF seed treatments. Crude protein and ether extract digestibilities and apparent biological value of fescue protein were not affected by presence of the fungus. The effect of INF seed on organ weights was inconsistent.(ABSTRACT TRUNCATED AT 250 WORDS)