1. The patterns of discharge of spontaneous GABAA-mediated inhibitory postsynaptic currents (sIPSCs), originating from the nucleus reticularis thalami (NRT), and their modulation by GABAB autoreceptors, were studied in rat thalamocortical (TC) neurones using whole-cell voltage-clamp recordings in brain slices. 2. sIPSCs were recorded in all ventro-basal (VB) and dorsal lateral geniculate (LGN) neurones. In VB neurones, in the presence of tetraethylammonium (TEA, 5 mM), these sIPSCs can occur in bursts at frequencies of either 0.1 or 1-2 Hz. In the presence of tetrodotoxin (TTX), these bursting activities are replaced by the continuous discharge of miniature IPSCs (mIPSCs), recorded in the absence of TEA, at a frequency of 4 Hz. The kinetic properties of mIPSCs were similar in VB and LGN TC neurones. 3. In VB TC neurones the GABAB receptor agonist (+/-)-baclofen, at a concentration of 0.05 microM, decreased the mIPSC frequency by 22% without affecting their amplitude distribution. Increasing the (+/-)-baclofen concentration to 1 and 10 microM caused similar reductions (41 and 47%, respectively) in the mIPSCs frequency: these values were significantly different from the one observed with 0.05 microM (+/-)-baclofen. In LGN TC neurones, where mIPSCs originate from both NRT and local interneurone terminals, 1 microM (+/-)-baclofen produced a 66% reduction in the mIPSC frequency. 4. The GABAB receptor antagonist CGP55845A (50 nM) not only blocked the baclofen-mediated decrease in mIPSC frequency, but also produced a 52% increase in the mIPSC frequency compared with control in three out of seven neurones. Application of CGP55845A (50-500 nM) alone produced a 77% increase in the mIPSC frequency in three out of nine VB neurones, and in the LGN, CGP55845A (100 nM) produced a 53% increase in four out of nine neurones. CGP55845A (100 nM) also reversibly increased the amplitude of evoked GABAA IPSCs by 74 and 57% in three out of three VB and three out of five LGN neurones, respectively. 5. Application of GABA (1.5-5 microM) decreased the mIPSC frequency in VB TC neurones by a similar extent (48%) as 1-10 microM (+/-)-baclofen. 6. In the presence of 100 microM Cd2+, (+/-)-baclofen still decreased the mIPSC frequency by about 40%, indicating that the effect of presynaptic GABAB receptor activation on spontaneous GABA release did not occur through a reduction of voltage-dependent Ca2+ currents. 7. Cd2+ (100 microM) decreased the amplitude of both mIPSCs and isoguvacine-induced current by 30 and 19%, respectively, indicating an effect of this divalent cation on postsynaptic GABAA receptors. 8. We conclude that GABAB autoreceptors are present on the GABAergic terminals within the thalamic sensory nuclei and that these receptors can be tonically activated by the ambient GABA.