As is the case with the anucleate mammalian erythrocyte, ionic composition in anucleate red cells of rainbow trout, Salmo gairdneri, is insulin-sensitive. By comparison with erythrocytes cultured in insulin-free medium, those exposed to insulin concentrations of 20, 200 and 2000 microU/ml for 2 and 6 h exhibited dose-dependent increases in potassium and water content coupled with reductions in the levels of sodium, magnesium and chloride. These observations suggest that the membrane of this type of erythrocyte possesses insulin receptors.