Monensin is a carboxylic ionophore that has been observed to increase cation permeability across the membrane of several cell types. Additionally, it is used commercially as an anticoccidial agent and has been found to increase feed efficiency in cattle. The objectives of these experiments were to determine the ability of monensin to stimulate cation (Na and K) transport across the bovine erythrocyte membrane and determine the effects of anion substitution on the action of the compound. Erythrocyte cation analyses revealed that all of the animals used in this study were low potassium (LK). Red cells were incubated in an artificial medium in the presence or absence of monensin, and cell sodium, potassium and water were determined at several time periods. It was observed that monensin stimulated the movement of sodium and potassium down their respective concentration gradients. Cell water content ("D") was observed to increase in response to an elevation in cell cation content. In synthetic media containing acetate, sulfate, citrate, thiocynate and gluconate substituted for chloride as the anion specie in the presence of monensin, there were measureable differences in intracellular sodium and water during the incubation period. The addition of DIDS to the control media containing chloride was observed to inhibit from 60 to 80 percent of the monensin-stimulated sodium movements. The results of this study show that monensin stimulates cation movements in bovine erythrocytes and anion substitutes may alter the action of this ionophore. Additionally, it was demonstrated that the action of monensin can be modified by inhibition of Band 3.