The formation of ammonia from histidine is optimum at p H 9. It does not take place at p H 2. Liver, kidney, spleen, pancreas, intestine mucosa, thymus, testis and muscle were tested for presence of histidases but they occurred only in the liver, in all the animals tested: dog, guinea pig, rabbit, goose, chicken and frog. Two of the three N atoms of histidine can be split off with ammonia formation. It is possible that in the metabolism of histidine by the organism, ammonia is sometimes formed; at other times urea.