(1) EMG investigations in 17 female manic-depressive patients under longterm treatment with lithium salts showed a reduction of the maximal motor nerve conduction velocity (NCV) and of the duration of isolated potentials (IPs) in six cases. These changes, however, were not clearly due to the influence of lithium only. Psychopathology, additional medication, and other variables may have been also responsible for these findings. (2) NCV and IPs as well as the concentration of electrolytes in serum and RBC were determined in seven healthy volunteers before, during, and after lithium administration. NCV was significantly decreased and the duration of IPs slightly increased after l week of lithium intake. The magnesium serum concentration increased steadily during the experimental period. (3) Thus, the possibility cannot be excluded that symptoms of fatigue and muscular weakness in lithium-treated patients might be caused at least partially by changes of the peripheral nervous-muscular function.