During stereotaxic penetrations (extracellular tungsten microelectrodes) in the thalamic nuclei CL, Pc, CM, VA, LD, MD, LP, RTh of curarized, artificially ventilated cats (local anaesthesia), all units encountered (145) were tested for their reactivity to fastigial stimuli (single or 5-200/sec square pulses, 0.1-0.5 msec, 1-10 V). Whereas none out ofthe 36 units found in association nuclei (LD, MD, LP) were influenced, about 75% of those located in the remaining nuclei proved to be reactive, the latencies of their responses varying from 3 to 10 msec. According to the response-patterns, the sensitive units can be ranged into three groups as follows: Group I, activation upon single-shock and low-frequency stimulation, blockade upon high-frequency stimulation with rebound-like effects; Group II, activation upon high-frequency stimulation only, never blockade; Group III, activation upon all kinds of stimulation tested (or upon single-shocks and low-frequencies only), never blockade. The proportion of Group I and Group II units was particularly high in the midline nuclei. All response-patterns were found with no qualitative differences in neuraxially-intact, in decorticate, and in encéphale isolé preparations.