In a study of the intradialytic behaviour of l-ascorbatemia during acetate and bicarbonate dialysis we noted the paradoxical increase in l-ascorbatemia during the first hour of acetate dialysis. During biofiltration, the intradialytic curve of l-ascorbatemia is similar to that in bicarbonate dialysis, even though an acetate dialysis bath is used. Thus the paradoxical rise of l-ascorbatemia during acetate dialysis may be due to the relative acidosis of the first hour.