A method is described for measuring ileal blood flow in the anesthetized (pentobarbital sodium) rabbit by the intraventricular injection of microspheres (15 micrometer) labeled with cerium-141 or chromium-51; with this method the amount of labeled microspheres lodging in the tissue is proportional to the blood flow. Blood flow to the ileal mucosa plus submucosa could be separated from flow to the ileal muscularis propria plus serosa by this technique. Simultaneous and sequential injections of radiolabeled microspheres gave similar measurements of ileal blood flow and did not affect ileal water absorption. Increasing ileal water absorption by treatment with the glucocorticoid methylprednisolone (3 mg/100 g per day for 3 days) increased blood flow to both compartments of the ileum and also to the colon, liver, and kidneys; methylprednisolone treatment did not alter blood flow when studies were performed before the methylprednisolone-induced increase in ileal water absorption had occurred. In contrast, intestinal secretagogues that induced both active ileal secretion (purified cholera toxin and serotonin) and passive ileal secretion (hypertonic mannitol) did not affect ileal blood flow. These studies indicate that increased ileal water absorption is associated with increased ileal blood flow, whereas intestinal secretion is not necessarily associated with an alteration in ileal blood flow.