The GABA_B receptors (GABA_B-R1 and GABA_B-R2) have been cloned in several mammalian species, and the functional receptor has been shown to exist as a heterodimeric complex. We have cloned guinea pig GABA_B-R1 and GABA_B-R2 receptor sequences and found that GABA_B-R1/R2 receptors are expressed in supraoptic (SON) and paraventricular (PVN) vasopressin neurones using in situ hybridization and immunocytochemistry for vasopressin (AVP). Therefore, we used both sharp electrode and whole-cell patch recording techniques to examine the effects of the selective GABA_B agonist baclofen on SON and PVN magnocellular neurones and to determine the coupling of the GABA_B receptor to effector pathways. Recordings were made in coronal hypothalamic slices from both female (ovariectomized) and male guinea pigs. In the presence of tetrodotoxin (TTX), baclofen hyperpolarized (V_max = 5.6 mV, EC₅₀ = 2.3 µM) SON magnocellular neurones (n = 27) under current clamp, or induced an outward current that reversed at E_K⁺, (I_max = 24.2 pA) in PVN magnocellular neurones (n = 33) under voltage clamp. Seventeen of the 24 biocytin-labeled SON magnocellular neurons were identified as AVP neurones, and ten of the 33 biocytin- labeled PVN neurons were identified as AVP or neurophysin-containing neurones, although all of the cells were clustered in the vasopressin-rich core. In the absence of TTX, baclofen activated an outward K⁺ current that hyperpolarized SON and PVN neurones and significantly reduced their firing rate. The outward current showed inward-rectification and was blocked by the K⁺ channel blocker barium and the GABA_B receptor antagonist CGP 35348. Therefore, GABA_B receptors are coupled to inwardly-rectifying K⁺ channels in SON and PVN magnocellular neurons and may play a prominent role in modulating phasic bursting activity in guinea pig vasopressin neurons.