Exposure of male guinea pigs to an atmospheric concn. of 170 p.p.m. of ammonia for 6 hrs./day, 5 days/week for as long as 12 weeks gave no evidence of chronic intoxication. However, such exposure for 18 weeks resulted in mild though definite pathological changes in spleens, kidneys, suprarenal glands, and livers. Hearts, lungs, stomachs, and small intestines remained normal.