Extravehicular activity (EVA) is an essential part of space missions. When International Space Station ALPHA (ISSA) is fully operational, EVA assembly, installation, maintenance and repair operations will become everyday repetitive work activity in space. Due to the high cost of EVA sorties, a logical step is to try to improve the productivity of the EVA astronaut in order to increase the amount of labor per EVA hour and to optimize the work/rest regime. This is a main goal of decompression protocol development. In order to allow the astronauts sufficient productivity within the EVA space suit it is necessary to operate the suit with an internal pressure lower than 1 atmosphere. The suit pressure of approximately 0.3-0.4 atmosphere (30-40 kPa or 220-300 mm Hg ) induces the risk of decompression sickness (DCS) by the formation of gas bubbles (GB) from excess nitrogen dissolved in the organism by breathing air at normal pressure in the space cabin atmosphere. To avoid this risk, the astronaut has to undergo a staged decompression with gradual pressure reduction or to use oxygen prebreathing for denitrogenation. This prebreathing procedure consumes considerable time ( up to several hours ) with corresponding impact on the operational options for EVA. The GB moving in the blood stream can be detected using on Ultrasonic Doppler Detector (UDD), which is capable of detecting GB in venous blood vessels and the heart cavities by frequency shift of the reflected ultrasound signal. The differential signal is used for an automatic evaluation of bubbling frequency and for semiquantative assessment of GB concentration and size. This technique has been developed for decompression tests by Biophyspribor Company at St. Petersburg, Russia. The UDD has been used in our laboratory decompression trials as an objective method of decompression stress assessment during EVA simulations during the last decade. Usually the Doppler method shows the gradual appearance of GB before any symptoms of DCS occur, so the UDD can be used to : a) determine the limit of allowable decompression, and b) act as an early warning signal to avoid DCS with the understanding that not everyone with GB develops DCS symptoms.The first goal of this study is to elaborate a set of decompression protocols which minimize the risks of DCS and at the same time maximize labour efficiency during repetitive EVA's. The second goal is verification of modern decompression protocols and individual DCS susceptibility using the automatic UDD system. All of these tasks are in accordance with the overall aim of the space programme of ensuring maximum safety, efficiency and performance for Russian and American astronauts. The results of 239 tests of automatic UDD system usage during EVA simulation (0.5 h oxygen prebreathing and 40 kPa operational pressure mode) are presented. The probability of DCS and GB are computed as a regression functions of supersaturation ratio, physical activity, duration of hypobaric exposure and individual anthropometric data of 38 test subjects.