To evaluate the effect of 30 min head-down tilt(HDT) at-6° on orthostasis, twelve anesthetized dogs(tilt group), weighing 8-14kg, were serially changed by body position as follow: from control supine position to –6° HDT as a model of weightlessness, from post-HDT supine to +70° head-up tilit(HUT) as the otrhostasis at every 30 min, other five dogs(control group) were continuously placed in the supine position for 60 min prior to orthostasis.