Results of acute toxicity studies on rare earth nitrates indicate that their acute toxicity is highly dependent on the route of administration. The nitrate salts were moderately toxic when given intraperitoneally to rats and mice and only slightly toxic when administered orally to rats. With the exception of the transition elements, for which the oral LD50 vales were greater than 5,000 mg/kg, the compounds exhibited an increase in toxicity with increasing atomic weight. The light lanthanons are highly toxic to rats by the intravenous route. The nitrate salts of cerium, praseodymium, neodymium, and samarium were from 7 to 11 times more toxic to females than to male rats. Erbium nitrate, a heavy lanthanon, did not show the marked sex difference in toxicity to rats. Rats were able to tolerate 1,000 mg/kg of the rare earth oxides given orally or intraperitoneally.