The clinicopathological syndrome of acute oral toxicity to parathion (98.5% technical) was determined in male albino rats fed for 28 days from weaning on a diet containing 3.5% protein as casein (group I), normal amounts of protein as casein (26% group II), and normal amounts of mixed natural protein as laboratory chow. The LD50±S.E. was found to be 4.86±0.31 mg/kg in group I, 37.1±4.9 mg /kg in group II and 23.4±5.4 mg/kg in group III. The LD50 in group I was significantly lower than that in group II which, in turn, was significantly higher than that in group III. Most deaths occurred at 10–35 hours in all three groups. The clinical syndrome of intoxication was essentially the same in all three groups and included signs of cholinergic stimulation such as diarrhea, sialarrhea and dacryorrhea, of stimulation of the central nervous system such as piloerection, tremors, and exophthalmos, alternating with signs of depression of the central nervous system such as hyporeflexia, drowsiness, ataxia, prostration and pallor. These signs were accompanied by anorexia, oligodipsia, hypothermia, loss of body weight, oliguria, proteinuria, hematuria and glucosuria. At autopsy there were found a very mild local irritant gastroenteritis, vascular congestion of many organs, a contracted spleen and, when death was delayed, gastric ulcers and degenerative changes in organs such as the liver and kidneys. Most organs were dehydrated and had lost weight. The survivors appeared normal by the 4th day but there was a persistent hematuria, glucosuria and proteinuria. Organ weights and water contents were mostly within normal limits at 2 weeks and 1 month but autopsy at these intervals occasionally disclosed healed or healing abscesses, indicating that resistance to infection had been lowered by parathion, and other residual changes such as a megacolon and hydrated muscle.