We sought to determine whether streamlining of portal venous blood occurs in normal anaesthetized rats under basal conditions and with variations in hepatic blood flow. We catheterized the ileocolic vein and injected 15 micron microspheres labeled with 85Sr and 141Ce into this vein and into the spleen, respectively. The hepatic lobar distribution of microspheres was studied in a group under basal conditions and after hepatic blood flow was increased (infusions of nitroglycerin or glucagon) or decreased (infusion of vasopressin or ligation of the superior mesenteric artery); this blood flow was measured with a constant infusion of indocyanine green. Measured results (expressed as proportion of total liver counts per minute) were compared with a reference group in which the portal vein of rats had been partially ligated 10 days prior to study and in which similarly injected microspheres that lodged in the liver were assumed to be completely mixed with portal blood. No differences were seen within groups and between the reference and experimental groups. We conclude that under these experimental conditions, portal venous blood flow appears to be distributed homogeneously between hepatic lobes.