This study aims to determine the pH that peripheral veins can tolerate. Intravenous nutrient solutions with different pHs (from 4.25 to 6.71) were infused into rabbit ear veins, and the veins were examined histopatologically. After 6-hr infusion at 10 mL/kg/hr, a commercial 2.72% amino acid/7.5% glucose solution with electrolytes (AG) caused obvious phlebitic changes, such as loss of venous endothelial cells, inflammatory cell infiltration, and perivascular edema, in all 6 rabbits because of its low pH (4.52) and high titratable acidity (22 mEq/L). The phlebitis was reduced when the solution was neutralized with NaOH to pH 5.93, and was almost eliminated when the pH was neutralized to 6.49. After 8-hr infusion at 15 mL/kg/hr, AF-adjusted pH to 6.30 caused slight phlebitic changes, but AG-adjusted pH to 6.71 scarcely caused any change. Furthermore, 24-hr infusion of the pH 6.49 solution caused no histopathological changes in 3 rabbits. These results suggest that the tolerance pH for the peripheral vein is about 6.5, and that an infusion solution does not cause phlebitis due to acidity if the pH is not lower than the tolerance pH.