In order to simulate daily episodes of high ammonia (NH₄⁺) absorption associated with the intake of diets with high content of non-protein N or rapidly rumen degradable protein, four wethers (42±3.4 kg BW), surgically fitted with indwelling catheters in the posterior aorta and splanchnic veins, were infused with 340 µmol NH₄⁺HCO₃^-/min into the mesenteric vein for 3 h, over 7 consecutive days. On the 7^th day, net mass transfers of NH4⁺, urea, glucose, lactate, ß-OH-butyrate and oxygen were measured across portal-drained viscera (PDV), liver and splanchnic tissues during the last 90 min of the NH₄⁺ infusion (NH₄⁺ treatment, AT). Measurements were repeated on the following day, after withdrawal of the NH₄⁺ infusion (Control treatment, CT). NH₄⁺ infusion increased liver NH₄⁺ uptake (+396 µmol/min; sed, 72; P=0.04) and urea production (+152 µmol/min; sed, 55; P=0.14), and oxygen consumption by the liver (+151 µmol/min; sed, 6; P=0.002), the PDV (+224 µmol/min; sed, 56; P=0.03) and the splanchnic tissues (+352 µmol/min; sed, 57; P=0.009). Net mass transfers of glucose, lactate and ß-OH-butyrate across the PDV and the liver, and the acid-base status of the animals were unchanged.