The effect of TRH on endotoxic shock in rats was studied, iv 0. 22-2 mg ?kg-1 TRH significantly reversed hypotension induced by iv coli E. endotoxin (40 mg ?kg-1) into rats and caused a 4. 2 kPa rise in mean arterial pressure (MAP). MAP after TRH administered could be stablized over a higher level than control for 30 min and maintained for 3 h during observation. Interestingly enough, the MAP rose gradually in TRH-treated rats as contrast with increasingly falling of that in control group during the late shock. TRH also improved 24 h sur-vival of shock rats. The dose-response relationship could be observed between 0. 22 ~0. 67 mg ?kg-1 TRH and disappeared over a highest dose (2 mg ?kg-1). It was shown that the best dose to reverse hypotension and to improve survival was 0. 67 mg ?kg-1 TRH. Naloxone 2 mg ?kg-1 showed the nearly same effect as 0. 22 mg ?kg-1 TRH in increasing MAP, but the former had higher 24 h survival of rats than the later.