The effects of hypocapnia [arterial PCO₂ (Pa_CO2) 15 Torr] on splanchnic hemodynamics and gut mucosal-arterial P_CO2 were studied in seven anesthetized ventilated dogs. Ileal mucosal and serosal blood flow were estimated by using laser Doppler flowmetry, mucosal PCO₂ was measured continuously by using capnometric recirculating gas tonometry, and serosal surface PO₂ was assessed by using a polarographic electrode. Hypocapnia was induced by removal of dead space and was maintained for 45 min, followed by 45 min of eucapnia. Mean Pa_CO2 at baseline was 38.1 ± 1.1 (SE) Torr and decreased to 13.8 ± 1.3 Torr after removal of dead space. Cardiac output and portal blood flow decreased significantly with hypocapnia. Similarly, mucosal and serosal blood flow decreased by 15 ± 4 and by 34 ± 7%, respectively. Also, an increase in the mucosal-arterial PCO₂ gradient of 10.7 Torr and a reduction in serosal PO₂ of 30 Torr were observed with hypocapnia (P < 0.01 for both). Hypocapnia caused ileal mucosal and serosal hypoperfusion, with redistribution of flow favoring the mucosa, accompanied by increased PCO₂ gradient and diminished serosal PO₂.