The influence of dietary iron on spontaneous activity was investigated in growing rats. Male rats weighing 97 +/- 10 g fed diets containing 4.6 (deficient), 19.9 (marginal), and 108.4 (adequate) mg Fe/kg diet for 8 wks had hemoglobin values of 52 +/- 6, 152 +/- 4, and 159 +/- 6 g/L, and liver nonheme iron concentrations of 0.79 +/- 0.24, 1.36 +/- 0.39, and 4.58 +/- 0.62 mumol/g, respectively. Body weights were affected only at the lowest iron intake. Diurnal activity was not reversed by iron status; all animals were more active during the dark cycle than the light cycle. Iron deficiency resulted in less time and frequency of horizontal, vertical, and stereotypic movements; less distance moved; and less frequent rotations. Movement speed of iron-deficient rats was faster during early iron deficiency. In the light, rats with marginal iron nutriture were generally more active than were iron-deficient or iron-adequate animals. In the dark, activity generally decreased with each reduction in dietary iron.