It is hypothesized that peritoneal fluid might be used as an alternative vehicle for monitoring the clinical chemistry of patients on peritoneal dialyses and save these patients the venesection and resultant blood loss. Peritoneal fluid was obtained before 47.2% of 106 dialysis treatments and compared with simultaneous serum samples. Very close correlation was noted for phosphorus, uric acid, blood urea nitrogen, creatinine, potassium, bicarbonate, and sodium (r greater than 0.82, p less than 10(-6)), and we conclude that peritoneal fluid may be useful for monitoring these chemistries. The peritoneal fluid concentrations of calcium and albumin were consistently 20% and 43% lower than the corresponding serum values. Because the peritoneal fluid is in equilibrium with the serum, it could also be used to measure protein binding of low molecular-weight substances in vivo that cross the peritoneal membrane. The results presented here fail to support the suggestion that urate is bound to serum proteins in vivo.