The study is an investigation into the form of the abdominal pressure reflex in normal individuals under isometric loading of the torso. Various directions of load are imposed, and both flexed and upright postures are adopted. The sensitivity of the pressurization to the applied moment is calculated, the form of the response being linear in each case. The results show that the response sensitivity varies with subject posture and mass and with the direction and site of loading. The highest sensitivities are in response to extensor moments and the lowest, when the torso is flexed. It would appear that the supportive influence of the trunk cannot be considered constant under all loading conditions, nor can a single explanation of its function be adequate under both extensor and flexor applied moments.