Arachidonic acid is released from cell membranes in response to receptor-dependent as well as receptor-independent stimulation in various cells, including cardiac myocytes. Arachidonic acid is converted to prostaglandins by cyclooxygenase and to leukotrienes by 5-lipoxygenase, metabolites which are very biologically active and modulate cellular functions such as platelet aggregation, smooth muscle contraction and neural excitation. The molecular mechanisms underlying their modulations are, however, still badly understood. Here, we report that the 5-lipoxygenase metabolites of arachidonic acid activate the pertussis toxin-sensitive G protein-gated muscarinic K+ channel (IK.ACh): arachidonic acid activation of IK.ACh was prevented by the lipoxygenase inhibitors, nordihydroguaiaretic acid and AA-861; leukotriene A4 and C4 activated IK.ACh. The activation occurred in pertussis toxin-treated atrial cells and ceased when inside-out patches were formed but the patches were still susceptible to stimulation by GTP and to inhibition by GDP-beta-S. These results indicate that arachidonic acid metabolites may stimulate the G-protein in a receptor-independent way.