Neuromuscular transmission was studied in the ulnar-hypothenar group in 55 patients with amyotrophic lateral sclerosis. A decremental response was found in 67.0%. The decrement was larger and present more often in muscles showing atrophy. In addition, muscles with frequent fasciculations showed a larger decrement than the ones with rare fasciculation. A temperature effect similar to that in myasthenia gravis was observed, with a reduction of the decrement following local cooling of the muscles. Administration of edrophonium chloride improved the synaptic defect. Posttetanic exhaustion was observed as well. It is thought that the defect of neuromuscular transmission is due to a decreased trophic function of the neuron followed by morphological changes at the endplate.