The effect of different levels (0, 10 and 20%) of O-beta-D-galactopyranosyl-(1-4)-O-beta-D-galactopyranosyl-(1-4)-D- glucose (4'-GL) on the bioavailability of sodium and potassium was studied in 18 male rats. Three 3-d metabolic balance studies were conducted during the 62-d feeding trial. These were between d 30 and 32 (first period), 45 and 47 (second period) and 60 and 62 (third period). Growth and food intake were not significantly different (P less than 0.05) between the control group and the 4'-GL-fed groups. In rats fed 10 and 20% 4'-GL diet, the fecal sodium excretion was significantly greater (P less than 0.05) in all three balance periods. Fecal potassium excretion was significantly greater (P less than 0.05) in the 10% 4'-GL-fed group in the second balance period and the 20% 4'-GL-fed group in all balance periods relative to the control group. Although not statistically significant, rats in the 4'-GL-fed groups exhibited a tendency for lower sodium retention and higher potassium retention compared with the control group. The cecal weight of rats in both 4'-GL-fed groups was significantly heavier (P less than 0.05) than that of the control group at the end of study. Sodium concentration in the cecum was significantly lower (P less than 0.05) in the 20% 4'-GL-fed group relative to the other groups.