Manipulation of the food supply can induce either intense hyperactive wheel running or a fatal activity anorexia in rats that is strongly analogous to that seen in humans. The abnormal behaviour is accompanied by alterations in the diurnal pattern of activity. As part of a detailed study of hyperactivity and anorexia, spontaneous wheel running by male rats was studied under three conditions: ad libitum feeding; restriction to 15 g of food per day; and restriction to a single 90-min meal per day. Ad libitum fed rats increased their running at the rate of 440 +/- 60 m/day per day, stabilizing after day 10 at 6045 +/- 3010 m/day. The running occurred in short bursts throughout the dark period and at the beginning of the light period. Rats restricted to 15 g/day increased their running at the significantly greater (p < 0.001) rate of 1230 +/- 120 m/day per day, reaching 12 200 +/- 4 090 m/day by day 10 and thereafter stabilizing at 13 600 +/- 4 160 m/day. The running was initially triphasic and confined to the dark period but eventually progressed to a biphasic pattern. The rats restricted to a single 90-min access period to food each day showed an even greater rate of increase in running, attaining 1930 +/- 288 m/day per day (p < 0.02 vs. 15 g/day group). These animals decreased eating and decompensated by day 4. The diurnal pattern of activity was disturbed from day 1 of the protocol, and by day 4 the rats ran essentially continuously throughout the daily cycle. The sensitivity to hyperactivity is a function of the severity of food restriction in this animal model of hyperactivity. It is paralleled by a marked disturbance of the diurnal pattern of activity, suggesting that the hyperactivity is related to a basic central nervous system dysfunction.