The intercalated cell masses are clusters of GABAergic neurons interposed between the basolateral and centromedial nuclear groups of the amygdala. Tract-tracing studies have revealed that the main projection sites of intercalated neurons are the central amygdaloid nucleus and the basal forebrain. Through these projections, intercalated neurons could influence the activity of widespread regions of the central nervous system. However, no data are available regarding their physiological properties because of the paramount methodological difficulties raised by the small size of intercalated cell masses. Here, we have investigated the spontaneous and evoked activity of intercalated neurons in unanaesthetized, chronically implanted cats. Extracellular recording sites were identified using stringent histological criteria. The intercalated cell masses were found to contain a population of neurons firing at much higher rates than commonly observed in neighbouring amygdaloid nuclei. Individual intercalated neurons displayed state-dependent changes in firing rates, but these varied from cell to cell. Most tested intercalated neurons displayed short-latency orthodromic responses to cortical shocks and were responsive to a variety of auditory stimuli. Considering that the vast majority of intercalated neurons use gamma-aminobutyric acid (GABA) as a transmitter, the presence of neurons with high spontaneous firing rates within the intercalated cell masses suggests that these cell clusters may provide a tonic inhibitory input to their projection sites. Moreover, the fact that the firing probability of some intercalated neurons could be altered by the presentation of sensory stimuli suggests that this inhibitory input can be modulated as a function of environmental contingencies.