Two types of potassium-selective channels activated by intracellular arachidonic acid or phosphatidylcholine have been found in neonatal rat atrial cells. In inside-out patches, arachidonic acid and phosphatidylcholine each opened outwardly rectifying potassium-selective channels with conductances of 160 picosiemens (I_K⋅AA) and 68 picosiemens (I_K⋅PC), respectively. These potassium channels were not sensitive to internally applied adenosine triphosphate (ATP), magnesium, or calcium. Lowering the intracellular pH from 7.2 to 6.8 or 6.4 reversibly increased I_K⋅AA channel activity three- or tenfold, respectively. A number of fatty acid derivatives were tested for their ability to activate I_K⋅AA. These potassium-selective channels may help explain the increase in potassium conductance observed in ischemic cells and raise the possibility that fatty acid derivatives act as second messengers.