The anorectic effect of CM 57373 in dogs and in rats food-deprived or with experimentally induced hyperphagia (cafeteria-diet hyperphagia and insulin hyperphagia) was compared to the effect of serotoninergic anorectic drug dl-fenfluramine. CM 57373 and dl-fenfluramine administered orally caused a dose-related reduction of food consumption by food-deprived rats (ID50 = 7.4 mg/kg and 2.5 mg/kg respectively). The oral ID50 in dogs was 2.4 mg/kg for CM 57373 and 1.1 mg/kg for dl-fenfluramine. This animal species tolerated CM 57373 better than dl-fenfluramine. The latter induced mydriasis, dyskinesia and reduced spontaneous activity. The anorectic effects of CM 57373 and dl-fenfluramine in cafeteria-diet hyperphagic rats were comparable. Tolerance to the anorectic effect developed in rats treated with both CM 57373 and dl-fenfluramine although tolerance was initially less pronounced with CM 57373 than dl-fenfluramine. The brain serotonin levels of cafeteria-fed rats were unchanged by CM 57373 throughout treatment whereas dl-fenfluramine decreased the monoamine levels starting from the 8th day. Both drugs reduced 5-hydroxyindolacetic acid levels. CM 57373 (7.4 mg/kg p.o.) and dl-fenfluramine (2.5 mg/kg p.o.) markedly reduced the overeating caused by insulin injection. These results indicate that CM 57373 shows several characteristics of drugs that act via serotonin to depress food intake in various animal species.