Influence of chronic administration of theophylline on feeding behavior in rats was studied in contrast with the saline controls under a 12 hr light-dark cycle. The results obtained were as follows: 1) total amount of food intake per 24 hr was not affected by either chronic administration of theophylline or saline administration; 2) theophyllinized rats, however, ate more food during light time (0900-2100) and less food during the dark (2100-0900); 3) the amount of food intake during the dark in the control group was approximately 80% of total intake, while that in the theophylline group decreased to below 60%; 4) increase in ratios of body weights in the theophylline and the control groups did not differ significantly throughout the entire experimental period. Feeding behavior of the rat is closely correlated with a daily light-dark cycle as was seen in the control group. The feeding circadian pattern disappeared in theophyllinized rats, i.e., the behavioral characteristic of a nocturnal feeder could not be preserved following theophylline administration. The disappearance of the feeding cycle may result from a phase shift of the cycle, which was induced by facilitatory effects of theophylline on food intake during light time.