The concentrations of insulin and thyroid hormones, tryptophan, electrolytes, urea, plasma proteins in the cerebrospinal fluid, and glucose in blood and cerebrospinal fluid in manic-melancholic patients were studied. As control groups served patients suffering from other psychiatric disorders as well as neurological and orthopedic patients. Apart from the blood values of thyroid hormones, the results showed no differences between the various diagnostic groups, neither in the abnormal states nor when recovered. For blood thyroxine and free thyroxine index, a statistically significant difference was seen in unipolar (melancholic) patients, namely a decrease concomitant with the clinical improvement. A tendency in the opposite direction of the thyroxine values was found in bipolar (melancholic) patients. In the manic group a marked decrease in the thyroxine values was obtained in the lithium-treated patients.