We compared the effects of lysophosphatidylcholine (LPC) and acetylcholine (ACh) on I_K(ACh), I_Ca and a non-selective cation current (I_NSC) in guinea-pig atrial myocytes to clarify whether LPC and ACh activate similar G_i/o-coupled effector systems.　I_K(ACh), I_Ca and I_NSC were analyzed in single atrial myocytes by the whole cell patch-clamp.　LPC induced I_NSC in a concentration-dependent manner in atrial cells.　ACh activated I_K(ACh), but failed to evoke I_NSC.　LPC also activated I_K(ACh) but with significantly less potency than ACh.　The effects of both ligands on I_K(ACh) were inhibited by intracellular loading of pre-activated PTX.　This treatment also inhibited LPC-induced I_NSC, indicating that I_K(ACh) and I_NSC induced by LPC are both mediated by G_i/o.　LPC and ACh had similar potencies in inhibiting I_Ca, which was pre-augmented by forskolin, indicating that LPC and ACh activate similar amounts of α-subunits of G_i/o.　The different effects of LPC and ACh on I_K(ACh) and I_NSC may suggest that LPC and ACh activate G_i/o having different types of βγ subunits, and that LPC-induced I_NSC may be mediated by βγ subunits of G_i/o, which are less effective in inducing I_K(ACh).