Factors regulating the activity of synaptosomal Na, K-ATPase have been found in the cytosol of nerve endings. The activatory effect of the factor increases in the presence of neurotransmitters regardless of their direct action on Na, K-ATPase. Synaptosomal Na, K-ATPase is not sensitive to the factor obtained from the cytosol of kidney tissue, or the cytosolic fraction obtained after sedimentation of microsomes. The effect of inhibiting low molecular ET(S) fraction on Na, K-ATPase activity is not mediated through noradrenaline, dopamine and serotonin as well by the system of secondary messengers. Factor stimulated by neurotransmitters activates the Na, K-ATPase system affecting the phosphorylating intermediates of the enzyme and putting the Na, K-ATPase system in the mode of simultaneous transport of Na and K ions.