In 26 dogs under chloralose anesthesia and neuromuscular blockade the hind limbs were vascularly isolated and perfused at constant flow. Electrical stimulation of the intact spinal ventral roots (L5,6,7) induced: 1. a decrease of the perfusion pressure (PP) in the ipsilateral limb (−42.9±4.1 mm Hg; mean ± SE); 2. a decrease of the PP in the contralateral limb (−36.4±5.1) and, 3. a fall of the aortic pressure (−14.3±1.5). Similar effects were observed upon stimulation of the peripheral stump of the sectioned ventral roots. These vascular effects were resistant to cholinergic and beta-adrenergic block, but were abolished by hexametonium and by the i.a. administration of antihistamines in doses which did not suppress the vascular reactivity to test doses of acetylcholine and isoproterenol. These experiments suggest the presence in these lower lumbar ventral roots of the preganglionic segment of an histaminergic pathway affecting vascular resistance in the hind limb of the dog.