The hemodynamic effects of high frequency jet ventilation (HFJV) at three different rates (60, 100, 200/min) and at rate 200/min combined with jet sighs 12/min (HFJV-200 + 12S) during two degrees of hemorrhagic hypovolemia were investigated in dogs. Also, the results were compared with those of intermittent positive pressure ventilation (IPPV). Two degrees of hypovolemia were produced by bleeding dogs until mean arterial pressure was 25% or 50% lower than basic value respectively. At both periods of hypovolemia, HFJV at rate 60 or 100 were found to have better hemodynamic effects than IPPV due to their lower airway pressures. However, HFJV at rate 200 or HFJV-200 + 12S did not demonstrate the same superiority because of their higher airway pressure, the latter even represented a tendency of worse hemodynamic effect than IPPV. The best cardiovascular effect was seen during HFJV at rate 100. Our study indicated that the hemodynamic effect of HFJV better than that of IPPV during acute hypovolemia can be seen only when proper ventilatory settings are chosen. Jet sighs at 12/min added to usual HFJV are not beneficial to circulatory function. It is recommended that HFJV at a rate below 200/min without jet sighs be used in patients who need respiratory support during acute hypovolemia or shock.