Two groups of anesthetized, splenectomized, and paralyzed dogs were hyperventilated (Vt 40 ml/kg). Normocapnia was maintained in one group (mean Paco2 37.6 mm"h'g, mean p"h '7.41) and respiratory alkalosis (mean Paco2 8 mmHg, mean pH 7.75) in the other. Splanchnic hemodynamic responses were similar in both groups. Average hepatic venous pressure increased from 3.2 to 6.4 mmHg in the normocapnic group and from 3.8 to 7.7 mmHg in the hypocapnic group. Average portal venous pressure increased from 10.7 to 12.0 mmHg and 10.8 to 12.7 mmHg in the normocapnic and hypocapnic groups, respectively. Mesenteric vascular resistance increased in 93 per cent of dogs. A decrease in functional intestinal capillary surface area during hyperventilation was indicated by a significant reduction in mesenteric Vo2 (from 15 to 11 ml/min, average 30 per cent), and a concomitant reduction in mesenteric O2 extraction ratio. Changes in mesenteric Vo2 were reflected in calculated splanchinic Vo2. Hepatic O2 uptake was essentially unchanged by tidal hyperventilation with or without hypocapnia.