Head-down tilt (HDT) at -6˚ has been commonly used as the experimental model in both man and animals to induce the blood shift toward the head or central protion of the body, demonstrating similar physiological effect encountered in the weightlessness in the orbital flight. There are few reports about the physiological response upon the cardiovascular regulatory system or the tolerance to the (-6˚) HDT within a relatively short period less than 1 hour. Therefore, the purpose of this study way to observe the effects of -6˚ HDT on cardiovascular system within 30 minutes and to evaluate early regulatory mechanism for simulated hypogravity. Ten mongrel dogs weighing 8-12 kg were anesthetized with the infusion of 1% α-chloralose (100 mg/kg) intravenously, and the postural changes were performed from the supine to the -6˚ head-down Position, then from the head-down to the supine (SUP), and each posture was maintained for 30 minutes. Blood flow (Q̇) through common carotid and femoral arteries were determined by the electromagnetic flowmeter. Mean arterial pressure (MAP), heart rate (HR), respiratory rate , and pH, Po₂, Pco₂ and hematocrit (Hct) of arterial and venous blood were also measured. The peripheral vascular resistance was calculated by dividing respective MAP values by Q̇ through both sides of common carotid or femoral arteries. The concentration of plasma epinephrine and norepinephrine was determined by Peuler & Johnson s radioenzymatic method. The results are summarized as follows: In the initial 5 minutes in -6˚ HDT, HR was significantly (p<0.05) increased and MAP slightly decreased. Although Q̇ and carotid peripheral artery resistance were not significantly changed, Q̇ through femoral artery was diminished and femoral peripheral artery resistance was elevated. In the SUP, the initial changes of MAP and HR were increased (p<0.05), but those of Q̇ and peripheral vascular resistance through both common carotid and femoral arteries were not significant. After 10 minutes of each postural change in both HDT and SUP, MAP was maintained almost equal to that of the level of pretilting control. During 60 minutes of both postural changes of HDT and SUP, Po₂ and Hct were not changed significantly. However pH tended to increase slowly and Pco₂ was gradually decreased. The pH and Pco₂ seemed to be related to the increased respiratory rate. Plasma epinephrine concentration was not changed significantly and plasma norepinephrine concentration was slightly decreased in the course of HDT and also at 10 minutes of SUP. However these concentration changes were statistically insignificant. From these results, it may be concluded that the effect of -6˚ HDT for 30 minutes on the cardiovascular system and plasma catecholamine levels of the dog is minimum and it is suggestive that the cardiovascular regulatory mechanism, possibly mediated by so called gravity receptors including baroreceptor and volume receptor, has been properly and adequately operated.