The first iodine-131-tagged compound used in evaluation of renal function was iodopyracet (Diodrast). et al. (1) in 1956 reported a study of renal function in rabbits with this compound and Winter (2) obtained iodopyracet I131 renograms in man. In both of these studies analysis of the results was attempted through a comparison of the curves obtained from the two kidneys in the same individual. Almost from the outset it was recognized that a variable percentage of the Diodrast being taken up by the liver and bound in the plasma, making this type of comparison difficult (3, 4). Caudal angulation of the scintillation probes and use of carrier Diodrast both attempted to obviate these difficulties. In our hands neither of these methods has been thought to be uniformly successful. Ortho-iodohippuric acid has recently become commercially available2 and we have subjected it to laboratory analysis in the dog, comparing it to iodopyracet. Method and Results Dogs were anesthetized with pentobarbital sodium administe...