Before and after 30-day head-down tilt (-8 degrees) the cardiorespiratory function of six healthy volunteers was assessed using an exercise test (aerobic workload on the treadmill that grew to the maximal level). After hypokinesia the maximal oxygen consumption decreased by 9.9% and total oxygen debt by 23% (P less than 0.05). The bioelectric activity of the heart showed a decrease of the TA wave by 34% and the TD wave by 30% (P less than 0.05). These changes give evidence that the functional capacity of the cardiorespiratory system declined and as a consequence the ability of the test subjects to perform sustained physical work of aerobic character also decreased.