Using a splanchnic nerve-spinal cord preparation in vitro that could spontaneously generate sympathetic nerve discharge (SND), we investigated the roles of intraspinal GABA_B receptors in the regulation of SND. Despite an age-dependent difference in sensitivity, bath applications of baclofen (Bac, GABA_B receptor agonist) consistently reduced SND in a concentration-dependent manner. The drug specificity of Bac in activation of GABA_B receptors was verified by application of its antagonist, saclofen (Sac) or CGP-46381 (CGP). Sac or CGP alone did not change SND. However, in the presence of Sac or CGP, the effects of Bac on SND inhibition were reversibly attenuated. The splanchnic sympathetic preganglionic neuron (SPN) was recorded by blind whole-cell patch-clamp techniques. We examined Bac effects on electrical membrane properties of SPNs. Applications of Bac reduced excitatory synaptic events, induced membrane hyperpolarizations, and inhibited SPN firing. In the presence of 12 mM Mg²⁺ or 0.5 µM TTX to block Ca²⁺- or action potential-dependent synaptic transmissions, applications of Bac induced an outward baseline current that reversed at -29 ± 6 mV. Since the K⁺ equilibrium potential in our experimental conditions was -100 mV, the Bac-induced currents could not simply be attributed to an alteration of K⁺ conductance. On the other hand, applications of Bac to Cs⁺-loaded SPNs reduced Cd²⁺-sensitive and high voltage-activated inward currents, indicating an inhibition of voltage-gated Ca²⁺ currents. Our results suggest that the activation of intraspinal GABA_B receptors suppresses SND via a mixture of ion events that may link to a change in Ca²⁺ conductance.