GABA_B receptor belongs to a family of G-protein coupled receptors known to be negatively coupled to adenyl cyclase in adult animals. Although its effects on K⁺ and Ca²⁺ channels have been well described, it is still controversial about the down-stream targets of GABA_B receptor modulated cAMP. In this study, we investigated the effect of GABA_B receptor on I_h current that can be directly modulated by intracellular cAMP level. In neurons of neonatal mice (P1-P11), I_h current was present in respiratory network. The I_h current density was markedly increased after first postnatal week. In 7 out of 25 neurons application of 5 mM baclofen decreased the amplitude of I_h current, while it increased the amplitude of I_h current in 18 out of 25 neurons. In addition, activation of GABA_B receptor showed concentration-dependent biphasic effect on I_h currents. Application of 5 mM baclofen significantly increased the amplitude of I_h current, while it decreased the amplitude of I_h current at concentration higher than 30 mM. Our data suggest that GABA _B receptor has significant effect on I_h current by modulating intracellular cAMP level. Thus, I_h could be one of the target of GABA_B receptor modulated cAMP. Furthermore, the biphasic effect of GABA_B receptor supports the hypothesis that there are at least two functional populations of GABA_B receptors that couple to either G_i-or G_s-mediated second-messenger pathways.