Mice were injected with crystalline jackbean urease in dosages of 0.4 units of enzyme activity per mouse at 0 and 12 hrs. One group was killed by decapitation at 24 hrs for analysis of brain DNA, RNA and blood ammonia. Stomach contents and rectal temperature were determined on similarly treated animals. Histological examinations of brain tissue obtained at 30–38 hr after initial injection showed increases in size and number of astrocytic nuclei and the astrocytes were typical in appearance of Alzheimer Type II cells. In the most severe cases there was spongy vacuolation of the neuropil. Mice showing these histological changes showed behavior which ranged from a small observable change to coma or coma with generalized convulsive seizures. About one half of the animals survived coma, but all showing convulsive seizures died. All animals showing behavior changes demonstrated significantly higher blood ammonia concentrations than those not showing behavior changes. The urease injected animals did not show a statistically significant reduction in brain DNA and RNA. Mice injected with an LD50 of urease (55 units/kg body wt) developed signs of intoxication earlier and the cellular changes in their nervous tissue were seen earlier than in animals injected twice with 0.4 units of urease. Gastric retention and hypothermia were observed in urease injected animals.