Mature rats were kept on protein-deficient diets to test the hypothesis that late-onset protein restriction results in deficits and to determine the feasibility of doing nutrition-behavior research with old naive animals. A 3% low-protein (LP) group and a 24% adequate-protein (AP) pair-fed control were used. Body weights and plasma protein concentrations were lower and exploratory behavior and motor coordination were poorer for LP rats. Both groups preferred the 24% protein diet. LP rats habituated slower and failed to overcome an initial (...) black preference on an oddity discrimination learning task. Nutrition-behavioral research with older rats is feasible, and late-onset protein restriction produces psychobiological deficits. (shrink)