This article sets out to show how physiological knowledge about sex/gender relates to power issues within sport. The sport physiology research at the Swedish School of Sport and Health Sciences (Swedish acronym: GIH) during the twentieth century is analysed in relation to the political rationality concerning gender at GIH and within the Swedish Sports Confederation during the same period. The analysis is constituted by Michel Foucault's notion of power--knowledge relations and regimes of truth. The construction of sex/gender in the physiological research changes over time. Comparative studies on the function of "sexual difference" during strenuous work, which, in hindsight, might be seen to restrict women's sport participation, was gradually displaced by a lack of interest in sexual difference, and later by a growing fascination with sexual difference from a "gender perspective" in terms of women being "different but equal" to men. This displacement goes hand in hand with a displacement of the political rationality concerning gender at GIH and within the Swedish Sports Confederation, where a pre-World War II strategy of excluding women's competitive sport participation, restricting women's physical exercise to gymnastics, was after 1945 followed by a strategy of including women. This was at first in the name of "women's right to do sport"--where the physiological research advocated this endeavour--and later in the name of "women's right to do sport on their own terms". However, the research was still being conducted based on the male physiology as the norm. (Contains 1 note.)