The nonionic radioopaque ultravist and the high-molar radioopaque verograffin were studied for their effects on the blood osmotic status of children with lower renal concentrating function. A total of 36 children aged 8 months to 12 years who had pyelonephritis, hydronephrosis and renal injury at their acute stage were studied angiographically under general anesthesia. The radioopaque was injected in a mean dose of 2 ml/kg for 2-3 sec. Ultravist was found to have a less osmotic action on the blood osmotic status than did verograffin. The changes in the detectable major blood osmotic parameters: sodium, potassium, glucose, urea, creatine were less pronounced. Plasma osmolality was moderately increased with ultravist and much higher than its normal values with verograffin at min 1 after its administration and at hour 2 of the study. Ultravist is preferable as a radiopaque used in children with decreased renal concentrating function.