In this study a simple rat model for evaluating the ultrafiltration by peritoneal dialysis solutions is described. Anaesthetised male Sprague-Dawley rats were injected intraperitoneally with dialysis solutions. Zero, 1, 3, or 6 h later, the dialysate volume was determined directly: the abdomen was carefully opened, the intraperitoneal liquid was withdrawn with a syringe and its volume was measured. Good recovery of dialysate, highly reproducible results, and the similarity between the ultrafiltration profiles in rats and published profiles in CAPD patients for control solutions, indicate that the model is valid. The model was then used to evaluate peritoneal dialysis solutions containing a mixture of glycerol and amino acids. Both osmotic agents are chemically compatible and these mixtures provide amino acids and carbohydrates in a single solution. For three mixtures, intraperitoneal dialysate volume were determined as a function of dwell time and the formulation with the desired ultrafiltration could be selected.