This paper embodies the results of endeavors to overcome certain disadvantages that have evidenced themselves since my original contribution1to excretion urography with iopax. For example, the volume administered was inconvenient, the dose large, the expense great, and the substance as such not representative of a product of normal metabolism. The underlying principle of the present investigations concerns itself with the utilization of a normal product of metabolism as an organic nucleus for combination with the radiopaque element necessary for the visualization of the urinary tract. The substance I now propose is sodium ortho-iodohippurate, a halogen derivative of a compound normally found in the human urine. In order to comprehend clearly the rationale for the development of this substance, the following considerations are worthy of note: 1. It has been known for a long time that the introduction of benzoic acid or its sodium salt into the animal organism