Among 1900 mice and 2600 rats of both sexes we studied the distribution between animals found dead and those sacrificed as moribund, during acute and chronic toxicity studies. Overall, the distribution was similar in both species and in both sexes (about 65% found dead and 35% sacrificed as moribund). There were no differences between gavage and dietary studies.In addition we analyzed the clinical signs and body weight of rats found dead or sacrificed as moribund, for 300 rats. Marked decrease in body weight, either acute or progressive, was the only distinguishing feature in animals found dead or sacrificed as moribund. These decreases occurred with a higher incidence in rats found dead or sacrificed as moribund, relative to the survivors at the end of the study, whereas the incidence and range of clinical signs were similar for all rats.We consider therefore that a decreased body weight and the rate of the body weight loss are the major criteria to consider in deciding whether or not rats should be sacrificed as moribund during the course of short-term and chronic toxicity studies.