The aims of this study were 1) to evaluate the responses of rhesus monkeys to food puzzle feeders set at three levels of task difficulty and 2) to determine whether exposure to food puzzle feeders altered the rates of self-injurious behavior and other abnormal activity. We examined the effectiveness of puzzle feeders in reducing self-aggression and other stereotypic behavior (e.g., pace, rock, eye-poke, hair pull) in 15 individually housed rhesus monkeys (8 of whom had a veterinary record of self-inflicted wounds). A pretest - test - posttest design was used in which the monkeys were provided with puzzle feeders only during the 6-week test phase. Food puzzle feeders consist of mazelike passageways with small openings through which monkeys can move a treat with their fingers until it reaches an opening large enough to extract the food. The maze can be altered to make the task easy or difficult. In this experiment, monkeys were exposed to 3 increasingly complex puzzle feeder tasks, each lasting for 2 weeks. Monkeys were observed not only when the puzzle feeder was filled in the early morning but also at several other time points throughout the day. On easy tasks, most monkeys emptied the puzzle feeder within a short period of time (i.e., within the first hour). On difficult tasks, monkeys tended to quit responding after the first hour even though much of the food remained. A comparison of the test phase with both the pretest and posttest revealed that puzzle feeders were only partially effective in reducing abnormal behavior. There was no effect on self-aggression; however, a reduction in active stereotypic behavior (pace, rock and bounce) was noted but only in the first hour of each daily exposure. These results suggest that although puzzle feeders increase exploration and manipulation, they have little effect on self-injurious behavior. Furthermore, effects on other kinds of abnormal behavior are transient, lasting only during the time when the animal is removing the food. Increasing the difficulty of the puzzle