Ketotic, insulin-requiring diabetes mellitus and a severe peripheral neuropathy developed in a previously healthy 25-year-old man several days after he attempted suicide with rat poison containing N-3-pyridylmethyl N'-p-nitrophenyl urea. Study of islet-cell function ten months after ingestion showed a reduced disappearance rate of intravenous glucose and depressed C-peptide response to intravenous glucose when compared with a normal control but no impairment of glucagon release after intravenous arginine stimulation. Nerve conduction studies demonstrated severe sensory and mild motor neuropathy. Quadriceps capillary basement membrane thickness was in the diabetic range. Because at least 15 similar occurrences have been reported to the manufacturer, this agent appears to be diabetogenic in man, probably causing beta-cell destruction. Niacinamide, which can prevent glucose intolerance in both streptozocin- and alloxan-treated animals and prevents death in rats given this rodenticide, may be a useful antidote.