Clorgyline, a selective MAO-A inhibitor, increased (5 times) rat pineal melatonin and N-acetyl-serotonin (NAS) content, and decreased 5-HIAA level by 80%. Deprenyl, a selective MAO-B inhibitor, did not change melatonin or other pineal indoles content. The data obtained show that inhibition of MAO-A but not B enzyme is responsible for pineal melatonin increase caused by MAO inhibitors. It is suggested that the stimulation of melatonin synthesis caused by MAO inhibitors may contribute to their antidepressive effect.