Iodoantipyrine I-131 is a radiopharmaceutical nonsteroid anti-inflammatory compound evaluated for coronary blood flow measure and as a total body water marker. In vivo results suggested that soon after injection of 4-I131 AP (Iodoantipyrine I-131) in rats , significant and increasing quantities of the I131 tracer were detached from the antipyrine molecule in a form which is more diffusible than 4-I131 AP. Evidence for a similar phenomenon was seen in other species. There were qualitative differences in the behavior of antipyrine and 4-I131 AP when injected in animals, and these may invalidate the use of 4-I131 AP as a substitute for antipyrine in studies of body composition.