This guideline is designed for use in acute ingestion tests using rodents, but is adaptable to other species. Although several accepted methods for determining LD50 values have been developed, many important determinants of toxicity are not represented either by these values or slopes of dose-response curves for lethality. These determinants are integral to an evaluation of acute toxicity and should be observed during the course of an acute toxicity study. Site and mechanism of action, early or delayed death, and recovery rate may be better indices of toxicity and hazard than LD50 values per se. Morbidity and/or pathogenesis may have more toxicological significance than mortality.