Epidemiological and animal model studies indicate that increased calorie intake increases the risk for colon cancer development. Previous studies in animal models restricted the calorie intake severely, and none of these studies have investigated a dose-response effect of different levels of calorie restriction on colon carcinogenesis. The present study was designed to investigate the effect of various levels of calorie restriction on colon carcinogenesis in male F344 rats fed the low and high fat diets and the effect of these diets on the activities of colonic mucosal and tumor ornithine decarboxylase (ODC) and protein tyrosine kinase. Starting at 5 weeks of age, groups of male F344 rats were fed the low fat or high fat diets ad libitum. At 7 weeks of age, all animals except the vehicle-treated groups were given s.c. injections of azoxymethane (AOM) (15 mg/kg body weight, once weekly for 2 weeks). Four days after the second injection, groups of animals were restricted to 90, 80, or 70% of total calories consumed by the high fat ad libitum group (i.e., 10, 20, and 30% calorie restriction, respectively). In the low fat groups, animals were restricted to 80% of total calories consumed by the low fat ad libitum group (i.e., 20% restriction). Thirty-six weeks after AOM injections, all animals were necropsied and colon tumors were used for histopathology and ODC and protein tyrosine kinase analysis. In the second experiment, the protocol was the same as above except that the animals were sacrificed 5 days after the second AOM injection and colonic mucosal ODC and protein tyrosine kinase activities were assayed. The incidence and multiplicity of colon tumors were significantly inhibited in animals fed the high fat 20% calorie-restricted and high fat 30% calorie-restricted diets, as compared to those fed the high fat ad libitum diet. The regression coefficient representing the dose-response effect of different levels of calorie restriction in both high fat groups is significant. Results also indicate that AOM treatment significantly increased the colonic mucosal ODC and protein tyrosine kinase activities. This stimulation was inhibited by feeding the calorie-restricted diets. ODC and protein tyrosine kinase activities were lower in the colon tumors of animals fed the calorie-restricted diets.