Children with various grades of renal insufficiency (CON group) maintained uric acid excretion over a range of glomerular filtration rate (GFR) from 27 to 160 ml/min*l. 73m2despite a decreased filtered load which was paralleled by glomerular filtration of uric acid. This was achieved by a compensatory decrease of net uric acid reabsorption (Tua) and an increasing fractional excretion of uric acid (FEua) with decreasing GFR. Although there was a decreased GFR in the group of children after transplantation (NTx group) there was no difference in Tua and FEua between the NTx group and the CON group. Uric acid transport was not affected in children treated with Cyclosporine (CyA)for nephrotic syndrome (NEPH group) compared to the CON group. Decreased fractional phosphate reabsorption in the NTx group suggests proximal tubule damage associatedwith disturbed uric acid handling. Under conditions of water diuresis hyperuricemia seen in NTx may result from an indirect effect of renal ischemic damage due to the transplantation procedure causing disturbance of proximal tubular uric acid (active) secretionlreabsorption.