A head-down tilt posture, the Trendelenburg position, which could facilitate venous return from the splanchnic organs and lower extremities, is recommended for the treatment of anaphylactic shock. However, few data of animal studies support its effectiveness. We examined the effects of a head-down tilt maneuver on anaphylactic hypotension in BALB/c mice and Sprague-Dawley rats. We measured systemic arterial pressure (Sap) and portal venous pressure (Pvp) in spontaneously breathing anesthetized animals sensitized with ovalbumin. At either supine (control) or a 30-degree head-down tilt position, anaphylactic hypotension was induced by an intravenous injection of antigen. In the control rats, an increase in Sap by 66 mmHg and a decrease in Pvp by 11.5 cmH(2)O were observed at 2.5 and 6 min, respectively, after antigen. In contrast, in control mice injected with antigen, Sap decreased similarly, but Pvp increased by only 4 cmH(2)O. A head-down tilt maneuver in mice substantially attenuated the antigen-induced decrease in Sap throughout the 60 min measurements, though it aggravated slightly, but significantly, only at the late phase of after 25 min in rats. We conclude that a head-down tilt maneuver attenuates anaphylactic hypotension in anesthetized mice and rats. These beneficial effects were smaller in rats than in mice probably because of substantial portal hypertension, which might prevent the head-down tilt-induced increase in venous return from the splanchnic vascular bed.