Occlusion of groups of vessels in the mesenteric vascular arcades of distal ileum for 48 h induced necrotizing enterocolitis lesions in low birth weight, spontaneously delivered, neonatal piglets. Lesion severity increased with numbers of adjacent groups of vessels occluded and with proximity to the ileocecal junction. A previously undescribed feature, "prepneumatosis," was confined to the lymphatic vessels of the submucosa and serosa. This feature closely resembled the position, shape, and distribution of classical pneumatosis intestinalis. Occlusion of vessels for 30 min followed by reperfusion did not induce any detectable changes 48 h later. Identical procedures (48-h occlusions) did not induce any detectable changes in 35-kg pigs.