SummaryTo determine whether solute transfer during peritoneal dialysis is age related and to identify those factors which might explain age‐related differences in dialysis kinetics, the peritoneal dialysance of [14C]urea (DU), [3H]inulin (DI), and the permeability index (DR= DI/DU) were examined in six puppies and five adult dogs. Exchange volume of lactated Ringer's (40 ml/kg) and exchange times (30 min) were identical in all studies. Theoretical calculations for urea dialysance for animals of differing body size were made. Assuming the existence of a similar functional peritoneal surface area per kg and the use of similar exchange volumes per kg and dwell times, theoretical values for the urea dialysance per kg for different sized animals were identical. The experimental studies demonstrated that DIper kg and DUper kg were higher in the puppies (0.146 ± 0.023 and 0.765 ± 0.054 ml/min/kg; &OV0398; ± S.E.) than in the adult (0.052 ± 0.01 and 0.462 ± 0.05 ml/min/kg) (P< 0.01). Also, DRwas higher in the puppies (0.187 ± 0.026), than in the adults (0.11 ± 0.015) (P< 0.05). The greater values for DIand DUper kg and DRin the young are best explained by the young having an increased peritoneal membrane permeability as well as an increase in functional peritoneal surface area relative to body weight. This increase in solute movement is independent of the dialysis mechanics used in an exchange and reflects agerelated differences in the intrinsic characteristics of the peritoneal membrane.