Summary The partial block of transmission at the neuromuscular junction in generalized myasthenia gravis manifests several characteristic features. 1. There is a slight degree of block to the passage of a single impulse. 2. Following the passage of a single impulse there is an increase in the degree of block which reaches a maximum about one second after the passage of the impulse. Ten seconds may be required before the degree of block decreases to its initial level. 3. The application of a train of impulses to the neuromuscular junction results first in a progressive increase in block to the passage of the impulses. This is followed by a transient decrease in block which in turn is followed by a progressive increase in degree of block. The magnitude of this increase in block increases as the stimulating frequency is raised. 4. Repetitive nerve stimulation is followed after a brief interval by a period of facilitation of neuromuscular transmission. This post-tetanic facilitation represents a decrease in the myasthenic block and is probably prejunctional in origin. The degree of post-tetanic facilitation is increased by increasing the duration or frequency of the tetanic stimulation. The post-tetanic facilitation of transmission of a single impulse is at the expense of the myasthenic subject's ability to transmit subsequent impulses. While such a study provides no direct evidence regarding the cause of the myasthenic block, it does afford a detailed description of this block. Such a description provides a basis upon which comparisons can be made between the myasthenic block and pharmacologically produced blocks; it gives a frame of reference against which drug-produced changes in the myasthenic block may be measured. The close resemblance between the neuromuscular block in myasthenia gravis and the block produced by d-tubocurarine in normal subjects has been pointed out. This indicates that the myasthenic block could be produced by a competitive (or curare-like) block.