Peritoneal dialysis was performed in 7 uremic children (4 months to 18 years). Rates of diffusion of Na, K, Cl, PO4, urea, creatinine, and uric acid were similar to adult diffusion curves. With the exception of uric acid diffusion, no age related differences were found. Peritoneal urea and creatinine clearnces paralleled the rate of exchage of dialysis fluid. Actual urea clearance values were within the range of clearance values obtained in adults dialysed at similar rates. Data to compare creatinine clearances in adults to children is not avialble over the range of dialysis volumes utilized in their studies. Estimation of urea production based on the average daily rise in BUN revealed similar production rates per kg body weight in all ages. These studies demonstrate no intrinsic age-related differences in the kinetics of peritoneal dialysis. Thus, there is no necessity to correct peritoneal clearances to cither 70 kg body weight or 1.73 M2, in order to compare children to adults. The greater ‘efficiency’ of peritoneal dialysis in children can be ascribed to a smaller body pool size in children relative to adults. Therefore, at a give clearance rate, the amount of material in the pool diminishes at a faster rate in children than in adults. Moreover, since peritoneal clearance increases as the rate of dialysis increases, no single clearance value for a given individual exists but rather a family of values related to the rate of dialysis. (Supported in part by NIH grants RR-5624. HD-2870 and FR-75.)