Electrocardiograms before and during lithium treatment were evaluated. In both 12 healthy volunteers who were given lithium for 7 days as well as in 20 out-patients, who had received lithium treatment for many years similar results were found: 1. Disturbances of the cardiac rhythm or heart failure did not occur. 2. Unspecific variations of the repolarisation were indicated in 75% of the long-term patients and 50% of the healthy volunteers during lithium treatment; these changes were dose-related. 3. The potassium concentrations in the serum remained within normal limits and showed no significant variations. 4. During therapy the long-term patients experienced a significant drop in the pulse-rate. This effect was apparently not dose-related, byt seemed to be conditioned by an incipient tachycardia caused by an additional psychotropic medication administered before the lithium application. In the healthy volunteers the pulse-rate remained stable during the experiment. These results are discussed in the context of the current literature of the subject.