Thoracic force-deflection characteristics as affected by impactor mass have been measured in the living subhuman primate as a step toward the analysis of cardio-thoracic injuries. Mechanisms of injury were investigated by subjecting Rhesus monkeys seated on a stationary freely movable sled to static and dynamic chest wall displacements (64 tests) by means of a cartridge-fired projectile equipped with a 3 in. diameter impact plate and an accelerometer compensated force transducer. Forces up to only 30 lb were developed in static press tests for 2 in. deflections but up to 700 lb for the same deflection during dynamic tests. Dynamic tests were run at four different initial impactor weights at 1.0, 1.7, 3.2, and 4.7 lb. Increasing the impactor mass tended to flatten the force-deflection curves which, for the lower impactor mass, showed one or two distinct peaks. These results complement the extensive work of other investigators in this area and indicate the need for additional primate studies.