Na,K-ATPase from duck salt gland and ox brain in the membrane-bound or solubilized form was studied by the radiation inactivation technique using ATP, CTP, GTP or p-NPP as substrates. The values of radiation inactivation size (RIS) were compared with the target size (TS) for the alpha-subunit of the enzyme obtained by an independent method as well as with analytical centrifugation data obtained for C12E8-solubilized enzyme. It was concluded that during ATP (CTP) hydrolysis the enzyme operates as an oligomeric structure; the complex formation requires the presence of K+ and adenosine triphosphate binding to the sites with a low affinity for the nucleotide. Specially designed experiments revealed that the degree of enzyme oligomerization increases with an increase in the microviscosity of the membrane lipid environment.