OBJECTIVE To investigate the possible effects of different concentrations of ionized sodium (NaI) on peritoneal ultrafiltration (UF) rate using lactate (Lac) and lactate/bicarbonate (Lac/Bic) dialysis solutions. METHODS Two random consecutive (after an interval of 48 hours) peritoneal equilibration tests (PETs) were performed in 13 patients (4 males and 9 females) on regular continuous ambulatory peritoneal dialysis (PD) treatment for at least 3 months. Two different PD solutions containing anhydrous glucose 3.86% were used: a 40 mmol/L Lac solution and a 15/25 mmol/L mixed Lac/Bic solution. Concentrations of total sodium (NaT) and NaI were measured by flame photometer and direct ion-selective electrode respectively. RESULTS Dialysate concentrations of NaT were not different during PETs using Lac and Lac/Bic. Dialysate concentrations of NaI in fresh PD solutions were different (133.3 +/- 1.7 vs 128.2 +/- 3.9 mmol, p < 0.0001); however, these differences disappeared just after the end of the infusion of the fresh solutions. Peritoneal UF rate was not significantly different during PETs using Lac versus Lac/Bic (609 +/- 301 mL vs 542 +/- 362 mL). The dialysate-to-plasma ratios of sodium concentrations at 60 minutes of the PETs (which are expressions of free water transport) were not different using Lac versus Lac/Bic (0.89 +/- 0.04 vs 0.89 +/- 0.04 respectively, p = 0.96). All the other classical parameters of the PET were not different between Lac and Lac/Bic. CONCLUSIONS The higher dialysate concentrations of NaI due to lower dialysate pH and consequently the higher effective osmolality of the fresh Lac PD solutions did not influence peritoneal UF rate, probably because of the fast reduction of NaI concentrations due to rapid correction of dialysate pH at the end of the infusion of Lac solutions into the peritoneal cavity.