Concentration-dependent effects of intracellular GTP on activation of the muscarinic K+ channel were examined in inside-out patches of cardiac atrial myocytes. The pipette solution contained 0.1 microM ACh. GTP (0.01-30 microM) and 0.5 mM MgCl2 were applied to the inside side of the patch membrane. K+ channels were activated with GTP concentration above 0.1 microM. Channel activation reached a maximal value with 1-3 microM GTP. It decreased at GTP concentrations larger than 3 microM, probably due to desensitization. The dependence of the open probability of the channel on intracellular GTP showed a sigmoidal relationship with a Hill coefficient of around 3. A positive cooperative effect of intracellular GTP on the K+ channel may play an important role in amplifying the signal from the membrane receptor to the K+ channel.