Thirty-two children were given either metrizoate or iohexol for urography in a double blind study. Mild to moderate adverse reactions were observed in all patients receiving metrizoate (15/15) and in 4 receiving iohexol (4/17). Alkaline phosphatase in urine was significantly increased 4 hours after the injection of both media, but had returned to pre-injection levels 16 hours later. The excretion of beta 2-microglobulin and albumin was not altered. In 9 children in the metrizoate group and 11 in the iohexol group the glomerular filtration rate (GFR) was determined before urography by the single injection 99Tcm-DTPA-technique and 3 to 4 hours after urography by measuring the plasma disappearance of the contrast medium with the x-ray fluorescence technique. No reduction of GFR was observed.