(1) Posterior hypothalamic stimulation causes a 36% reduction in colonic perfusion in the puppy with a simultaneous rise in vascular resistance. Neither effect is seen in the presence of hypoxia. (2) Hypoxia alone produces a 17% increase in colonic perfusion with a 33% decrease in resistance. In the presence of PHS, hypoxia is associated with an 83% increase in perfusion and a 43% reduction in resistance. (3) The data suggest that sympathetic overactivity may play a role in the genesis of the ischemic lesion of neonatal necrotizing enterocolitis, while hypoxia may be operative in the development of the hemorrhagic lesions. This concept is discussed in relation to other hypotheses regarding the genesis of this disease.