The Na,K-ATPase has been only partially purified from nervous tissue, yet it is clear that two forms (and ? +) of the catalytic subunit are present. ? is a component subunit of the glial Na,K-ATPase, which has a relatively low affinity for binding cardiac glycosides and ? + has been identified as a subunit of the Na,K-ATPase which has relatively high affinity for cardiac glycosides. The ? + form may also be sensitive to indirect modulation by neurotransmitters or hormones. The ratio of ? + /? changes in the nervous system during development, and ? + appears to be the predominant species in adult neurones. Changes in Na,K-ATPase activity have been associated with several abnormalities in the nervous system, including epilepsy and altered nerve conduction velocity, but a causal relationship has not been definitively established. Although the Na,K-ATPase has a pivotal role in Na(+) and K(+) transport in the nervous system, a special role for the glial Na,K-ATPase in clearing extracellular K(+) remains controversial.