The occurrence of irritability has been described in most text-books as a symptom in manic and depressive states. The causation of this symptom and the circumstances in which it is observed, have not received much close attention. Most authors put the states of irritability under the heading of mixed states. They cannot, however, be regarded as such in a strict sense, because the effect of irritability brings into the picture something new which cannot be derived from a mixture of the basal manic-depressive symptoms.