Fluid and electrolyte movements across the ruminal epithelium of sheep were studied using the temporarily isolated rumen technique. The sheep were all subjected to the following treatments: (1) fed sheep (fed twice daily), after rumen emptying, received rumen contents from a fed sheep; (2) food-deprived sheep (two meals were omitted), after rumen emptying, received rumen contents from a food-deprived sheep; (3) fed sheep, after rumen emptying, received rumen contents from a food-deprived sheep; and (4) food-deprived sheep, after rumen emptying, received rumen contents from a fed sheep. Food deprivation led to an increased Na concentration of the rumen fluid while K and Cl concentrations, as well as osmolality, decreased. Plasma Na and osmolality decreased. During the 40 min after the rumen contents were exchanged no net movements of water occurred. Then the sheep were given an intraruminal load of saline which gave rise to a significant net absorption of fluid from the rumens of those sheep which had received rumen contents from fed sheep. The change in composition of the rumen contents after food deprivation impaired the absorption of Na and water from the rumen. Furthermore food deprivation reduced the Na absorptive function of the ruminal epithelium, but not the water permeability.