1. The circulatory response following acute loss of 26% of the blood volume was examined in unanaesthetized rabbits. The groups of animals studied were normal rabbits; adrenalectomized rabbits; animals subjected to prolonged treatment with guanethidine in which peripheral adrenergic nerve transmission is blocked, but which can reflexly liberate adrenal medullary hormones; animals subjected to combined adrenalectomy and guanethidine treatment with no functional adrenergic effectors; in each case with or without administration of atropine. The responses of animals with section of the carotid sinus and aortic nerves were also examined.