We investigated the effect of hot bath and infrared radiation on tonic muscle discharges due to hypertonia in the affected upper extremity of hemiplegic patients using surface electromyography (EMG). Subjects were 15 hemiplegic patients with cerebrovascular diseases. The subjects were selected on the basis of the presence of considerable rigidospasticity in the biceps brachii muscle (BBM). Hot bath (42°C) was applied for more than 3 minutes in the supine position with the cubital joint fixed in flexion in 11 subjects. Infrared radiation (300 watts) was applied to the flexor side of the affected upper extremity in the same posture in 13 subjects. Surface EMGs of the BBM and triceps brachii muscle (TBM) were recorded bipolarly with waterproof disc electrodes before, during, and after the trials of hot bath and infrared radiation. The recorded EMG was rectified and integrated, and then converted into sequential pulses. The amount of EMG was calculated as the number of pulses. The EMG of the affected side was normalized as a percentage of the amount versus that of the unaffected side in maximum voluntary contraction. The EMGs of the BBM and TBM showed tonic muscle discharges in all subjects in the resting state. The EMGs of the BBM and TBM averaged 6.4% and 1.4% before hot bath and 3.3% and 1.2% before infrared radiation. The EMG of the BBM in the third 1 minute during hot bath decreased by 58.8% (p<0.01) and increased slightly after the trial. The EMG of the TBM during hot bath increased insignificantly, and decreased by 25.3% (p<0.05) in the second 1 minute after the trial. The EMGs of the BBM and TBM in the third 1 minute during infrared radiation decreased by 31.9% (p<0.01) and 9.3% (p<0.05), and the decrease persisted after the trial. The results demonstrated that thermotherapy, especially hot bath, decreases the tonic muscle discharges due to rigidospasticity.