The inhibitory postsynaptic potentials (IPSPs) evoked in neurons of the rat ventral geniculate nucleus (vLGN) by electrical stimulation of the optic tract and the action of GABA and baclofen on the same cells were studied using intracellular recording technique in an in vitro slice preparation. A short latency short duration IPSP always followed the monosynaptic excitatory postsynaptic potential (EPSP). This IPSP reversed in polarity at about -65 mV and was reversibly blocked by bicuculline (50 microM) thus indicating that it represents a GABAA receptor-mediated IPSP. No long-lasting IPSP was evoked in vLGN cells by stimulation of the optic tract, while in the same slice, long-lasting GABAB IPSPs were routinely recorded in the dorsal lateral geniculate nucleus. GABA applied by ionophoresis evoked a hyperpolarization that had a reversal potential close to -70 mV and was antagonized by bicuculline. Baclofen hyperpolarized vLGN neurons and its action was reversibly blocked by the selective GABAB antagonist phaclofen (1 mM). In the presence of bicuculline GABA also produced a hyperpolarization that had properties similar to that evoked by baclofen. These results indicate that, although functional GABAA and GABAB receptors are present on vLGN neurons, stimulation of the optic tract evokes only GABAA but not GABAB mediated IPSPs. The lack of long-lasting GABAB IPSPs could explain the absence of long-lasting inhibition observed in vLGN neurons in vivo following stimulation of the optic tract.