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Objective: The energy requirements of cows and sheep are met by ruminal fermentation of cellulose into short chain fatty acids (SCFA), which are extensively absorbed by forestomach epithelia. While disappearance rates from the rumen support a permeability sequence of butyrate > propionate > acetate, efflux rates into the blood suggest an inverse order. Basolateral efflux has recently been proposed to occur via a large conductance anion channel. The object of the current study was to characterize this channel in more detail. Methods: Isolated cells of the ruminal epithelium were studied with the patch clamp technique. Results: The permeability of the whole cell membrane for halides showed a loose correlation to the hydration energies, with the permeability for fluoride clearly lower than that for the other halides (P(F)/P(Cl)=0.45 ± 0.04, n = 10; P(Br)/P(Cl)=1.01 ± 0.08, n = 10; P(I)/P(Cl)=1.05 ± 0.08, n = 10). The permeability of the channel for the polyatomic anions tested decreased with molecular size (P(acetate)/P(Cl): 0.42 ± 0.01, n = 16; P(propionate)/P(Cl): 0.39 ± 0.01, n = 47; P(butyrate)/P(Cl): 0.36 ± 0.02, n = 32; P(gluconate)/P(Cl): 0.26 ± 0.01; n = 39), possibly reflecting a decrease in binding energy due to greater partial charge distribution or other steric effects. Single channel measurements confirm the whole cell permeability sequence, with the conductance for propionate (114 ± 10 pS) lower than that previously measured for chloride (350 ± 7 pS) or acetate (142 ± 7 pS). Conclusions: Ruminal epithelial cells express a large conductance anion channel with a moderately weak field strength selectivity site for monoatomic anions. The passage rates of the polyatomic anions of SCFA decrease in the order acetate- > propionate- > butyrate-, reflecting the order of appearance in portal blood. The low passage rate of butyrate- through the channel may explain why this SCFA is so extensively retained and metabolized within the ruminal epithelium.