This study examined the effect of different types of loss of control on the quality and quantity of the frustration response in 4- to 6-month-old infants. To establish an expectancy, all infants received 4 minutes of contingency training in which infants were presented with slides and music after they performed a pulling response with their right arm. The frustration period, which followed initial contingency training, lasted 2 minutes. Three frustration conditions were investigated. These were: (1) loss of outcome, in which arm responses no longer produced outcomes and outcomes never occurred; (2) reduced control, in which outcomes consistently occurred with every third arm response; and (3) loss of control, in which arm responses no longer produced outcomes and outcomes occurred independently of arm responses. The frustration period was then followed by a reinstatement of the initial contingency. Changes in arm responses and incidence of expressions of anger were the measures of frustration. Results indicated that infants in the loss of control condition exhibited more angry expressions than infants in the other conditions; and, unlike infants in the other two conditions, exhibited a decrease in arm responses. Results suggested that loss of control was a critical determinant of anger in young infants. (MM)