We examined the stimulating effect of Substantia Innominata pars anterior (SIa), during the waking state, on the 'central' part of the Mediodorsal nucleus of the thalamus (MD), combining electrophysiological and anatomical techniques in restrained, undrugged, unanaesthetized cats. Thalamic MD units were recorded, after electrical stimulation of the Substantia Innominata, at 1 Hz, with a single pulse or short trains of four pulses. Responses were studied by poststimulus histograms. In about 64 of the 84 recorded MD neurones (76%), stimulation of the Substantia Innominata, during the waking state, induced a brief cell excitation, followed first by prolonged inhibition of firing and then by a strong excitatory rebound discharge; after this comes a second sequence of inhibition and excitation, of decreasing amplitude. After stimulation of the Substantia Innominata, the MD units tended to start a repetitive discharge at 4--7 Hz. To investigate the connections of Substantia Innominata cells upon the areas where MD units were recorded we injected horseradish peroxidase wheat germ agglutinin (WGA-HRP), combined with immunohistochemistry for glutamic acid decarboxylase (GAD) and choline acetyl transferase (ChAT). Of the total population of retrogradely labelled cells in the Substantia Innominata 53% were GAD positive while less than 16% were ChAT positive. The GAD positive MD-projecting cells in the Substantia Innominata were triangular to fusiform and small to medium in size. These findings indicate that GABAergic input from the Substantia Innominata may contribute to increasing the hyperpolarizing inhibitory pressure on MD cells in the 'central' part during slow wave sleep (SWS).