Objective. To investigate the pattern of decrement in the muscles of patients with myasthenia gravis (MG) and Lambert—Eaton myasthenic syndrome (LEMS). Material and methods. Twenty-seven patients with MG and 39 patients with LEMS were studied using low frequency repetitive nerve stimulation (3/ sec). Results and conclusion. The decrease of safety factor of neuromuscular transmission was equal in both groups. At the same time, a significant difference in the decrease of pattern of the amplitude compound of muscle action potential (CMAP) was found. In LEMS, by contrast with MG, another sequence of amplitude variability of CMAP from the second stimulus to the first and from the fifth stimulus to the fourth was noted. In LEMS patients, progressive decrement, manifesting by increasing ratios of late A9/A1 to early A4/A1 was found, whereas the MG patients showed retrogressive decrement expressed by the reduction in decrement ratio (from late to early). These differences reflect the mechanisms and status of acetylcholine mobilization and release from the axon terminal.