A study has been carried out to verify the nephrotoxic effect of hyperosmotic substances used as a contrast medium in radiology employing the enzymuria technique (NAG). The research was carried out in 202 patients, some of whom diabetic, suffering from various pathologies and subjected to perfusion urography by inoculating an iodate contrast medium (isopaque). The result showed that NAG urinary activity grows significant in the 24-36 hours subsequent to X-ray examination in both diabetics and nondiabetics although the increase is greater in the former. After 5-7 days, enzymuria returns to normal in nondiabetic patients but remains high in the diabetic.