I. The uric acid formed when slices of tissue were incubated in oxygenated sols. was determined by the Folin colorimetric method. A non-purine, heat-stable precursor of uric acid is present in the tissues of the pigeon, notably in the muscles, liver, and kidneys. The substance is formed enzymatically during the incubation of liver or kidney surviving tissue or tissue powder. It may be extracted from, but is not formed in, muscle tissue. The actual transformation of the precursor into uric acid requires living kidney tissue. Optimum pH for the transformation in phosphate buffer is 7:1. The quantity of uric acid formed may be increased by the addition of amino acids or ammonium carbonate, but is not affected by urea or by 3-carbon compounds such as glycerol, or lactic, malonic, and tar-tronic acids.[long dash]II. In the hen and goose, as in the pigeon, the formation of uric acid involves two steps, (a) the formation of a precursor, and (b) its transformation into uric acid. The precursor is formed enzymatically, surviving tissue being unnecessary, but living tissue is required for converting it to uric acid. In contrast to observations on the pigeon, both liver and kidney tissue of the hen and goose can effect the change. The precursor in the tissues of the pigeon, hen, and goose appears to be the same substance.