Mice were given 1 to 67 daily injections of whole plasma from a control subject and a patient with the Lambert-Eaton myasthenic syndrome (LES), and the parameters of neuromuscular transmission in diaphragm muscles were studied in vitro. In animals that received 41 to 59 injections of LES plasma, mean quantum content of the endplate potentials, as measured under the conditions of low [Ca2+]0 (1.2 mM) and high [Mg2+]0 (10 mM), was reduced to 30% of the control, or 40% of the value found in normal untreated mice. When [K+]0 was elevated from 5 mM to 17.5 mM, the frequency of spontaneous miniature endplate potentials increased by a factor of 98 and 105 respectively in the control and normal mice but only 39 times in the LES plasma recipients. Despite these presynaptic abnormalities, no evidence of postsynaptic deficit, nerve conduction failure, or inability of the muscle fibers to produce normal action potentials was found. These electrophysiologic features in mice thus represent a reproduction of the human condition, supporting the concept of humoral involvement in the pathophysiology of LES. Passively transferred LES in mice is a faithful animal model of the human disease which will be useful in exploring the molecular basis of the presynaptic impairment.