Abstract: Suppose that the increase of the gas bubble volume is bound tobe followed by the increase of the incidence of decompression sickness(DS).It can be deduced that relative volume of gas bubbel in the tissue(the ratiobetween the bubbel volume and the tissue volume,V_b/V)has linear relationto the radio of N_2 tension in the tissue before decompression and that afterdecompression(P_(N2)/P_(dN2)).N_2 tension after decompression(P_(dN2))equals thesum of environmental pressure and synthetic pressure(P_s).P_s depends onactivity level.When the activity level increases,P_s decreases,V_b/V increasesand incidence of SD increases.In accordance with this hypothesis and experi-mental data on SD,the curves of the relation between O_2 prebreathing timeand the pressure in space suits are obtained.