The superior mesenteric blood flow in the post-absorptive state was studied with a dye-dilution technique using indocyanine green in 37 patients with apparently normal small bowel function. All the patients except 2 had been, or were to be, treated because of malignant disease, mostly rectal carcinoma. The investigations were performed in connection with portography. The dye was injected into the superior mesenteric artery as a single injection during continuous sampling of blood from the superior mesenteric vein, catheterized through the reopened umbilical vein. The blood flow was calculated from the dye-dilution curves obtained with a modified Stewart-Hamilton formula. As recirculating dye appeared in the later part of the dye-dilution curves, the appearance of the recirculating dye was timed in 4 patients. The hepatic extraction of the amount of dye usually injected, 1.25 mg, was 60%. The superior mesenteric blood flow averaged 706 ml/min (397 ml/min/m-2 body surface) or 12% of the cardiac output. The appearance time, mean transit time, vascular resistance and vascular volume were also determined.