Modulation of the activity of potassium and other ion channels is an essential feature of nervous system function. The open probability of a large conductance Ca(2+)-activated K+ channel from rat brain, incorporated into planar lipid bilayers, is increased by the addition of adenosine triphosphate (ATP) to the cytoplasmic side of the channel. This modulation takes place without the addition of protein kinase, requires Mg2+, and is mimicked by an ATP analog that serves as a substrate for protein kinases but not by a nonhydrolyzable ATP analog. Addition of protein phosphatase 1 reverses the modulation by MgATP. Thus, there may be an endogenous protein kinase activity firmly associated with this K+ channel. Some ion channels may exist in a complex that contains regulatory protein kinases and phosphatases.