Earlier work confirmed that the fundamental lesion of superphosphate poisoning is an acute toxic tubular nephritis in which both phosphate and fluoride may play a part but their respective roles could not be determined. In this study, sheep poisoned by sodium fluoride (NaF) were compared with sheep poisoned by superphosphate containing approximately 1.5% fluoride. The LD50of NaF was in the range 100 to 300 mg/kg (45 to 135 mg F/kg). This range is of the same order as the amount of fluoride in a toxic dose of superphosphate (70 to 90mg F/kg).