AbstractWe have found that cation transport in red cells from chick embryos is stimulated by the hormone epinephrine and that this response develops as the embryonic definitive cells mature. Sodium efflux and potassium influx are significantly stimulated (50%) by epinephrine in red cells from embryos incubated ten days or longer, whereas cation fluxes in erythroid cells from 8‐ or 9‐day embryos are stimulated little or not at all.The effect of epinephrine may be mediated by cyclic AMP as adenylate cyclase activity in membranes isolated from embryonic red cells is only slightly stimulated at nine days, but the response increases as the cells mature to a maximum of about 180%. Also the stimulation of cation transport by epinephrine is blocked by propranolol, but not by phentolamine. Although the younger cells respond poorly to epinephrine, cyclic AMP significantly stimulates transport.The enhancement of cation fluxes by epinephrine or cyclic AMP occurs even in the presence of ouabain. Since both K influx and Na efflux are enhanced by these agents, their action is most likely on some form of the “Na‐K” pump which is not ouabain sensitive resulting in a significant increase in the maximum velocity of the pump. We suggest the hypothesis that there are two classes of “Na‐K” pump in these embryonic cells. One pump is similar to that found in many erythrocytes including mammalian cells in that it selectively pumps potassium in and sodium out, is ouabain‐sensitive, and is primarily involved in maintaining intracellular cation concentrations. The second pump is enhanced by epinephrineviacyclic AMP, is not inhibited by ouabain, and may have lower ion selectivity. This hormone sensitive pump activity is lost as the cells mature, a process which is completed when the animal is fully grown and no longer has significant numbers of embryonic cells i