The authors tested the effects of varying cage size on the behavior of 10 female and 10 male Macaca fascicularis by singly caging them for 2 weeks in each of 5 cage sizes, ranging from approximately 20% to 148% of regulation size. Behavior in the regulation cage size, a size 23% smaller, and a size 48% larger did not differ in any analysis. Locomotion was significantly less in the 2 smallest cage sizes. Abnormal behavior occurred only 5% of the time, did not increase as cage size decreased, and did not change significantly over nearly 3 years. Disruption of the normal activity budget in the laboratory environment proved to be a useful indicator of psychological well-being. Moving to a new room and, to a lesser extent, moving into a new, clean cage, regardless of size, was associated with disrupted sleep the 1st night and suppressed activity, especially self-grooming, the next day.