Preservation of astronaut crew health during an exploration mission to the Moon or Mars will be crucial to mission success. The likelihood of a life-threatening medical condition occuring during a mission to Mars has been estimated by NASA to be 1% per year (Johnston, 1998; Johnston, Campbell, Billica, & Gilmore, 2004). Since basic life support is a vital skill in critical care medicine, plans must be in place for cardiopulmonary resuscitation in both microgravity and hypogravity (i.e., on the surface of the Moon or Mars). Following the design of a body suspension device to simulate a hypogravity environment, subjects performed external chest compressions in 1G, 0.17G (Lunar), 0.38G (Mars), and 0.7G (Planet X). Chest compression adequacy was assessed by means of rate and depth. Heart rate immediately before and after three minutes of chest compression gave a measure of rescuer fatigue. Elbow flexion was measured using an electrogoniometer in order to assess the use of arm muscles to achieve chest compressions. This study found that the mean depth (Lunar and Mars) and rate (Mars) of chest compression was below American Heart Association recommendations during hypogravity simulation in the female group. Furthermore, elbow flexion proved to be significantly greater during Lunar and Mars hypogravity simulation than that of the 1G control condition, suggesting that upper arm force may be used to counter the loss of body weight in an attempt to maintain adequate chest compression under these conditions.