The effect of corticosteroids on electrolyte movement was studied in an everted sac model from distal rat colon. Corticosterone had a significant stimulatory effect on sodium and fluid transfer in a dose-related manner. At the low dose of 10(-9) M corticosterone increased sodium absorption by 29.4 microEq X g-1 X h-1, a value that was statistically greater than the effect of 10(-9) M dexamethasone. At the higher concentration of 10(-7) M the effect of corticosterone on sodium movement was twice that of aldosterone or dexamethasone in equimolar concentrations. By contrast, the effect of corticosterone on potassium movement was biphasic: at lower concentration (10(-9) M) a significant net secretion occurred, whereas potassium absorption was observed at 10(-7) M corticosterone. Dexamethasone significantly increased net potassium secretion, meanwhile the effect of aldosterone on potassium movement was not significantly different from controls. These data suggest that the native glucocorticoid, corticosterone, exerts regulatory control of colonic fluid and electrolyte function.