The acid-base characteristics of two peritoneal dialysis solutions containing either lactate or acetate are compared and the time course of changes in intraperitoneal pH following instillation into the abdominal cavity is measured. The concentration of titratable acid (cTA) is 5.58 mmol/l or 7 times as high in solutions containing acetate as in those containing lactate (0.79 mmol/l). The buffer capacity, -dcTA/dpH, is 11.43 and 1.82 mmol/l, respectively. Following intraperitoneal instillation of 1.5 liter of the solutions, the time course is 2-3 times as long before intraperitoneal pH reaches 7 using acetate (18 min) as when using lactate (7 min). The above mentioned difference in acid-base characteristics as well as an individual acetate intolerance is supposed to be the cause for the development of abdominal pains and peritoneal irritation observed in some patients using acetate-containing solutions. 123 mmol/l of sodium bicarbonate is to be added to the acetate solution to raise the pH value from 5.6 to 7.4. Neutralization using sodium bicarbonate will thus result in sodium intoxication of the patient. The use of lactate instead of acetate for peritoneal solutions is advocated.