In our previous rat studies (Kidney Int. 1991; 39: 608-617) we evaluated peritoneal clearances (Cp) representing near exclusively diffusive CO2 transfer: for isosmotic (0.37% dextrose) and hyperosmotic (15% dextrose) solutions with pH 7.2-7.3, CpCO2 were 1.20 +/- 0.08 and 1.84 +/- 0.04 ml/min, respectively. In the present studies we have compared Cp of CO2 gas and HCO3- in anesthetized rats (n = 22) using solutions with dextrose contents as mentioned above but with pH 6.5 or 7.6; we have also evaluated how much Cp CO2 measurements obtained with solutions at these pH values differ from Cp shown earlier with the solutions of pH 7.2-7.3. Cp of CO2 gas and HCO3- were significantly higher under hyperosmotic than isosmotic conditions. The use of solutions of pH different from 7.2-7.3 resulted in higher Cp of CO2 gas: with isosmotic solutions of pH 6.5 and 7.6 mean increases were 25 and 75%, respectively; with hyperosmotic solutions respective increases were 45 and 134%. We conclude that dialysis solution osmolality and pH significantly change Cp of CO2 gas and HCO3- in the rat. For evaluation of peritoneal blood flow from diffusive transfer parameters of CO2 gas, smaller overestimation can be expected when dialysis solution pH is slightly under-than overadjusted compared to blood pH.