The intravenous injection of purified scorpion toxin (tityustoxin, TsTX) into unanesthetized rats induces a severe systemic hypertension followed by a hemorrhagic edema of the lungs. The edema is focal or diffuse, whereas the hemorrhage is always focal and less prominent than the edema. Anesthesia of the rats prevents the appearance of pulmonary edema. It seems likely that this protective action of the anesthesia is due, at least in part, to an interference with the hypertension induced by TsTX. The pulmonary edema is prevented by bilateral adrenalectomy, guanethidine or phenoxybenzamine. It is suggested that the edema depends on a sympathetic-adrenal discharge and that catecholamines released by TsTX act on alpha adrenergic receptors. The mean kininogen content of the rat plasma, 1 h after TsTX injection, is not significantly different from that found in the control animals. The possible role played by kinins and other mediators in the early phases of the pulmonary edema induced by TsTX is under investigation.