We1have reported that the function of muscles in patients with myasthenia gravis is temporarily improved by the administration of guanidine hydrochloride. The present discussion gives a more detailed account of our experience with the use of guanidine for the treatment of this disease. Altogether we have now observed the action of the drug in seven persons with myasthenia gravis. In five instances outlined in the following case reports we have been able to observe the patients for a considerable period. One of the other two patients was a Negress with mild myasthenia gravis complicated by marked hypothyroidism. In this instance a typical response to guanidine was elicited but medication has not been continued further. The other person had had myasthenia gravis for several years but was in a state of practically complete remission following thymectomy. This woman gave the same slight response to guanidine seen in some normal