Since mother loss produces depression as a short-term effect, it may produce depression, or a predisposition to depression, as a long-term effect. We shall study (1) the long-term effects of a month-long maternal separation in pigtail and bonnet monkey infants living in multigeneration social groups, and (2) the factors likely to be involved in the process of producing these effects. We expect that this information will suggest ways to prevent depression or to ameliorate it when it occurs. Maternal separation will be carried out either in the home pen or in a cage in the presence of other caged monkeys. Subjects will then be studied until adulthood. Control subjects will not be separated but will be studied in an identical manner. Long-term effects will be sought (1) in measures of subsequent normal development, (2) in abnormal behavior patterns, especially depression, and (3) through tests designed (a) to assess cognitive function, (b) to measure relative social dominance, and (c) to produce stress and frustration, and demonstrate vulnerability to helplessness and depression. The study should clarify the relationship between object loss and helplessness in the causation of depression.