In rats, the hemorrhagic gastric erosions produced by ethanol, and the fatal hemorrhagic hepatic necrosis induced by phalloidin, were significantly reduced by regular somatostatin, but not by derivatives devoid of -SH containing cysteines. These effects of the hormone were abolished in animals which received, in addition, the sulfhydryl blocker n-ethylmaleimide before the toxic chemicals. Thus, somatostatin exhibits organoprotection dependent on endogenous sulfhydryls.