We studied the respiratory output in 5 subjects exposed to parabolic flights (gravity vector 1, 1.8 and 0 G_z ) and when switching from sitting to supine (legs bent at the knees). Despite differences in total respiratory compliance (highest at 0Gz and in supine and minimum at 1.8Gz), no significant changes in elastic inspiratory work were observed in the various conditions except when comparing 1.8Gz compared to 1Gz (subjects were in the seated position in all circumstances). although the elastic work had an inverse relationship with total respiratory compliance that was highest at 0Gz and in supine posture and minimum at 1.8Gz. Relative to 1Gz, lung resistance (airways plus lung tissue) increased significantly by 52% in the supine but slightly decreased at 0Gz. We calculated for each condition the tidal volume changes based on the energy available in the preceding phase and concluded that an increase in inspiratory muscle output occurs when respiratory load increases (e.g. going from 0 to 1.8Gz), while a decrease occurs in the opposite case (e.g from 1.8 to 0Gz). Despite these immediate changes, ventilation increased going to 1.8 and 0Gz (up to 23%), reflecting an increase in mean inspiratory flow rate, tidal volume and respiratory frequency, while ventilation decreased ( -14%) shifting to supine posture (transition time about 15s). These data suggest a remarkable feature in the mechanical arrangement of the respiratory system such that it can maintain the ventilatory output with small changes in inspiratory muscles work in face of considerable changes in configuration and mechanical properties.