Behavioral and EEG changes induced by brain stem compression and the effect of TRH were studied. The compression was given for 1 to 6 min by inflating a balloon chronically implanted on the dorsal surface of the cat brain stem in the 4th ventricle via cisterna magna. Within 10 to 36 sec after the start of the compression, the cats turned sideways and became motionless in a spastic extension of four legs, and thereafter all reverted to a normal position, after 45 to 120 min, although slight movements or head-up position was observed in some animals. The cortical EEG patterns observed after the compression were initially a brief rush of low amplitude-fast waves (EEG arousal) followed by a flattened and/or spike pattern, and subsequently these shifted to high amplitude-slow waves with or without an accompanying EEG arousal. These behavioral EEG alterations were remarkably improved by i.v. administration of TRH as follows: eight of 12 cats with 1 mg/kg and one of 4 cats with 0.5 mg/kg promptly changed from the lateral to a crouching or abdominal position, and thereafter never turned sideways again. Partial recovery such as movements of forelegs, struggling or head-up in the lateral position, rolling or slight shift of position was also observed within several min in three cats with 1 mg/kg as well as in two cats with 0.5 mg/kg. Furthermore, TRH induced a dose dependent, persistent EEG arousal in all cats. These results show that TRH ameliorates deterioration in behavior and the EEG, this deterioration being similar to clinical states of disturbance in consciousness induced by compressing the brain stem.