Supersensitivity to depolarization produced by succinylcholine and resistance to pancuronium were observed in paretic muscles of a patient with a right frontoparietal tumor. The abnormal sensitivity to relaxants is compared with observations reported in patients with myasthenia gravis and hemiparesis. We hypothesize that upper motoneuron dysfunction may be followed by the appearance of "new" junctional receptors, which may occasional a supersensitivity to depolarization and a poor affinity for both curare and anti-acetylcholine-receptor antibodies. A decrease in acetylcholinesterase activity of "decentralized" muscles should also be considered.