Excessive dietary Fe is known to be toxic, but the extent of neurobiological involvement is not clear. In the present study male weanling rats were fed diets containing Fe at 35 (control), 350, 3500, or 20000 ppm for 12 wk. An Fe-deficient group (4 ppm) was included for comparison. Rats were tested for behavioral and body weight changes at various times after initiation of diets, and liver and brain nonheme Fe were measured at term. Excess dietary Fe, primarily at 20000 ppm, significantly decreased activity, habituation, reflex startle, and conditioned avoidance response performance, and enhanced prepulse modulation of startle. Body weights were also markedly decreased. Fe-deficient animals showed similar behavioral effects but more moderate body weight changes. Liver nonheme Fe varied directly with dietary levels. Whole-brain nonheme Fe was significantly reduced in Fe-deficient animals but increased only at the 20000-ppm level. Homeostatic mechanisms appear to regulate whole-brain Fe more effectively under conditions of dietary Fe overload than under conditions of Fe deficiency. The behavioral changes associated with dietary Fe overload may represent secondary consequences of systemic toxicity.