Lactate (L)-containing dialysate has a low pH, responsible for its poor biocompatibility. Dial-ysate-containing bicarbonate (B) with a physiological pH is available. We compare the peritoneal functional effects of these two solutions. Eight patients were studied two weeks apart using two consecutive peritoneal kinetic tests to determine convective and diffusive mass transfer coefficient (MTC) capacities for different solutes. Both L and B solutions were identical, except for pH and buffer content. Peritoneal equilibration values at four-hour dwell time were similar for all solutes, except for urea (B: 0.91 +/- 0.05 vs L: 0.87 +/- 0.06, p < 0.05). Peritoneal MTC (B vs L) values for urea (19.6 +/- 6.6 vs 18.2 +/- 4.5), creatinine (8.8 +/- 4.8 vs 7.8 +/- 3.5), phosphate (7.7 +/- 4.6 vs 6.0 +/- 2.2), and potassium (13.3 +/- 5.6 vs 11.7 +/- 5.0) were slightly higher for B (NS). Glucose-MTC was slightly lower for B (6.2 +/- 3.3 vs 7.5 +/- 3.2, NS). Ultrafiltration (UF) was lower for B (drained volume: 2120 +/- 204 vs 2443 +/- 285 mL after 4 hr, p < 0.05). Higher transperitoneal calcium [11 +/- 22 L vs -3.1 +/- 11.0 (positive balance) mg, p < 0.05] and sodium (48 +/- 30 L vs 13 +/- 20 mEq) removal were evident. With L, the loss of 43.0 +/- 6.3 mEq of bicarbonate appeared. The linear regression analysis of MTC values showed higher r coefficients for the bicarbonate solution. These results suggest that bicarbonate dial-ysate improves the diffusion capacity of the peritoneum with respect to lactate.