Plasma immunoreactive insulin has been assayed in blood samples drawn simultaneously from portal and peripheral venous blood following an injection of insulin into the ligated intestine of rabbits. The portal venous blood flow has also been measured using a perivascular electromagnetic flow transducer to estimate the amount of insulin absorbed. Following an injection of insulin with a dose of 100 U/kg, portal vein insulin concentration increased to the peak of 330 μU/ml in the ligated jejunal loop experiments, and 1280 μU/ml in the ligated ileal loop experiments. Both in the jejunal and in the ileal loop experiments, the peak insulin concentration in the portal vein was twice that in the peripheral vein. A significant correlation between portal and peripheral vein insulin responses (as reflected by the area) was noted. By multiplying the differences in the two venous insulin concentrations by the portal venous blood flow, the peak absorption was observed at a rate of 7.0 mU/kg/min in the jejunum, and 21.2 mU/kg/min in the ileum. During the 3 hr experimental period, insulin was absorbed from the jejunum to the extent of 0.4 to 1.6% of the dose given. 1.3 to 4.0% of the insulin injected was absorbed from the ileum. (Endocrinology93: 1371, 1973)