The hemodynamic response to missile injury was studied in the hind legs of dogs. Spherical projectiles were used and velocity and mass were varied.The regional arterial blood flow to the injured thigh was recorded and the energy transfer to the tissues determined. In addition the pressure within the aortic arch was recorded immediately after the impact of the missile. Missile penetration caused a marked peak of flow followed by a less pronounced elevation of the flow level for at least 30 minutes. The magnitude of the peak was related to the amount of energy transferred to the tissues. The origin of the circulatory changes is discussed. Shock waves of considerable magnitude were recorded within the aortic arch in connection with the bullet penetration.