Two mechanisms regulating the activity of the sodium transport system of the cell membrane are being studied. The salt gland of the domestic duck is being used for these studies because it presents a number of unique experimental advantages. One mechanism provides very rapid changes in sodium pump activity and appears to be mediated by cyclic GMP and calcium. We have presented evidence that cyclic GMP directly activates the sodium pump (NaK-ATPase) in intact cells. We plan to extend these studies to a cell free system to obtain information as to how cyclic GMP activates the pump. The second mechanism regulates pump transport capacity by controlling the synthesis of NaK-ATPase. We have shown that the large increases in NaK-ATPase content of the glands that occurs after ducks are maintained on salt drinking water is due to an increased rate of protein synthesis. We next intend to determine whether the increase in enzyme synthesis depends on a mechanism controlling the rate of messenger RNA translation or on a mechanism that increases the amount of messenger RNA in the salt gland secretory cells.