The skeletal muscles of individual unfasted [female] rats were analyzed for moisture, lipid, Na, K, Ca and Mg. The animals were studied in the following conditions normal with no therapy; normal treated with desoxycorticosterone acetate; adrenalectomized treated with NaCl; adrenalectomized treated with desoxycorticosterone acetate, and untreated adrenalectomized in varying degrees of insufficiency. Pooled sera from individual animals in each of these groups were analyzed for Na, K, Ca, Cl and non-protein-N. The K content of the muscles of animals in adrenal insufficiency was increased and very variable. The avg. total base of the muscles of all of the groups was remarkably constant with the exception of the group in adrenal insufficiency. Below the critical K level there was an orderly relationship between K and the other ions. Above this level in adrenal insufficiency, this relationship was abruptly changed. Below the critical K level, for every 10 meq. of K gained by the muscle, on the avg., 3.5 meq. of Mg were lost. Desoxycorticosterone acetate, when administered under the conditions described, caused the entrance of small quantities of Na into the muscle cells of normal animals and of larger quantities of Na (and perhaps of a little Ca) into the muscle cells of the adrenalectomized animals. This hormone and also NaCl prevented the rise in muscle K which would have occurred in the adrenalectomized animals in its absence. A possible mechanism for this effect is discussed. There was no evidence of increased permeability of the muscle cells in this group of animals. The probable importance of the ratio of K to Na in the muscle cell has been pointed out.