Extracellular recordings of 105 neurones in the cat's somatosensory thalamus were obtained with carbon fibre-containing multibarrel micropipettes. The responses of cells to natural stimulation of cutaneous or deep structures were characterized and the responses to electrical stimulation of primary somatosensory cortex were determined. Receptive fields were mapped and the functional properties were examined before and during the microiontophoretic administration of glutamate, gamma-aminobutyric acid (GABA) and bicuculline methiodide (BMI). Modality and submodality properties of all cells tested apparently remained unchanged qualitatively, despite all pharmacological interventions. BMI lowered the response threshold of a majority of the 48 cells tested for this variable, although almost 25% responded with elevated thresholds. BMI changed the temporal properties of the responses of both thalamocortical relay neurones and of presumed interneurones. Discharges evoked by natural stimuli and by electrical stimulation of the cortex were prolonged and their pattern was altered. Decreases in the frequency of bursts of discharges were often observed with BMI, and these bursts were invariably prolonged and the interspike interval profiles were altered. Receptive field size changes were observed only in 8 of 48 neurones. For two of these, the field size decreased, while for the others there were small increases.