The effects of three anti-ulcers drugs on the temporal distribution of food intake and of the two parameters, meal size and meal frequency, were studied in ulcerated and non-ulcerated rats exposed to light-dark (LD 12:12) cycles. Experimental ulceration with indomethacin reduces the amplitude of meal frequency and brings the acrophase forward, compared with non-ulcerated animals. These effects were reversed by the oral administration of either ranitidine, sucralfate or pirenzepine along with the food. However, the administration of either pirenzepine or sucralfate alone to non-ulcerated rats is accompanied by significant (P less than 0.05) changes in the circadian patterns of meal size and meal frequency without the total daily food intake being affected in any way (pirenzepine treatment caused large intake of food during the light period while sucralfate treatment resulted in marked food intake during the dark period). The results indicate that circadian modification of meal patterns in the ulcerated rats are attributable to indomethacin-induced gastrointestinal mucosal injury and anti-ulcer medications.