An integrative approach emphasizing both psychological and physiological components in force perception has started to emerge in motor psychophysics. In this experiment, the relation between isometric force (produced by the elbow flexors~ and perceived force was examined over a range of forces maintained until maximal endurance. A contralateral-limb matching procedure in which subjects estimated the force of a sustained, constant force contraction by contracting their unfatigued arm at regular intervals was employed. A linear increase in perceived force was observed during the fatiguing contractions, the rate of which depended on the level of force exerted. The sensation of force at maximal endurance was also found to vary with the force exerted. Based on the similarity between these results and those derived from electromyographic studies, we propose that observers use the efferent input to the muscle in preference to its afferent responses in judging the force of muscular contractions.