A method was developed to measure simultaneously the O₂ consumption (VO) by the whole animal and by the hepatic portal vein-drained organs (PVDO), including the gastrointestinal tract, spleen, and pancreas in conscious 3.5- to 4-month-old swine. The method was used to determine (i) the effect of feeding on hepatic portal vein blood flow rate (Qpv) and VO by PVDO and by the whole animal, and (ii) the significance of PVDO on the oxidative demand in the pig. Chronic cannulas were placed in the hepatic portal vein, carotid artery, and ileal vein. The Qpv was determined by an indicator dilution technique employing continuous constant infusion of 1% ρ-aminohippuric acid into the ileal vein. The VO₂ by PVDO was estimated by multiplying Qpv by arterial-portal vein O₂ difference measured with an arterial-venous O₂ difference analyzer connected to the carotid artery and portal vein cannulas. Whole animal VO₂ was measured with an open circuit indirect calorimeter. In seven pigs (3.5- to 4-month-old, 37.4 ± 0.8 kg) trained to be fed once daily, feeding (1.2 kg of feed mixed with 1.2 liter of H₂O) caused postprandial (6 hr) Qpv to increase more than 34 ± 15% above the preprandial value of 34.5 ± 4.2 ml · min^-1 · kg^-1 body wt. The postprandial VO₂ by PVDO was elevated more than 46 ± 12% above the value of 1.52 ± 0.20 ml · min^-1 · kg^-1 body wt observed during the preprandial period. Whole animal VO₂ increased 45 ± 9 and 33 ± 7% above the preprandial value of 6.23 ± 0.57 ml · min^-1 · kg^-1 body wt for the first 6 hr and the 7 to 12 hr after feeding, respectively.