The hypothesis that changes in gravity acceleration (a(g)) affect the linear relationships between oxygen consumption VO2 and mechanical power (w ) so that at any w, VO2 increases linearly with a(g) was tested under conditions where the weight of constant-mass legs was let to vary by inducing changes in a(g) in a human centrifuge. The effects of a(g) on the VO2/w relationship were studied on 14 subjects at two pedalling frequencies (f(p), 1.0 and 1.5 Hz), during four work loads on a cycle ergometer (25, 50, 75 and 100 W) and at four a(g) levels (1.0, 1.5, 2.0 and 2.5 times normal gravity). VO2 increased linearly with w. The slope did not differ significantly at various a(g) and f(p), suggesting invariant mechanical efficiency during cycling, independent of f(p) and a(g). Conversely, the y-intercept of the VO2/w relationship, defined as constant b, increased linearly with a(g). Constant b is the sum of resting VO2 plus internal metabolic power (E (i)). Since the former was the same at all investigated a(g), the increase in constant b was entirely due to an increase in E (i). Since the VO2 versus w lines had similar slopes, the changes in E (i) entirely explained the higher VO2 at each w, as a(g) was increased. In conclusion, the effects of a(g) on VO2 are mediated through changes in E (i), and not in w or in resting VO2.