In the CNS, there are multiple isozymes of the sodium and potassium ion- stimulated adenosine triphosphatase (Na,K-ATPase) that have differences in affinity for Na+, ATP, and cardiac glycosides. Three forms of the catalytic subunit (designated alpha 1, alpha 2, and alpha 3) are known to be derived from different genes, but little is known of the cellular distributions of the proteins or their physiological roles. Isozyme- specific monoclonal antibodies permitted the immunofluorescent localization of the 3 Na,K-ATPases in the rat CNS, and markedly different patterns of staining were seen. All 3 isozymes were detected, singly or in combination, in 1 or more neuronal structures, while both alpha 1 and alpha 2 were detected in glia. Many different neuroanatomic structures or cell types stained for more than 1 isozyme. Even when a structure or region stained for more than 1 isozyme, the pattern of staining was frequently dissimilar, suggesting complex differences in gene expression and cellular localization.