We have studied the appreciation of heaviness in two groups of subjects; in 8 patients with varying degrees of unilateral 'upper motor neuron' weakness but without sensory symptoms or signs, and in 10 volunteers during partial curarization of the forearm and hand. In all experiments the subjects matched a reference weight lifted on the weak side with a variable weight on the strong side to provide an objective indication of the heaviness of the reference weight. Patients matched the weights of tennis balls filled with mercury or weights lifted by extending the index finger. Weights were judged as heavier when lifted by the weakened side both in the hemiparetic patients and in normal subjects weakened with curare. In both the hemiparetic patients and the subjects weakened with curare peripheral sensory information would signal the forces exerted in lifting the weight; however, both groups of subjects clearly placed more reliance on the effort that was being put into the contraction than on peripheral tensions achieved. The concept that we rely in part on a sensing command to the motoneurons when estimating weight agrees with observations by Holmes on patients with unilateral cerebellar dysfunction, and with recent physiological experiments.