It has been suggested that housing of laboratory primates in two-tiered racks adversely affects the psychological well-being of those primates housed on the lower row. Excessive darkness and its consequences are among the factors suggested to account for the supposed diminished well-being of lower-row inhabitants. Additionally, two-tiered housing has been suggested to introduce unacceptable variation into experimental designs, potentially necessitating additional subjects and/or invalidating results. Only recently have data been published to address these issues, but all studies have involved small numbers of subjects. In the present study, we compared the behaviour of 45 yearling rhesus macaques (Macaca mulatta) housed in upper-row cages with the behaviour of 48 yearling rhesus macaques housed in lower-row cages during a year of single housing. There were no significant differences across cage locations for time spent performing behaviours indicative of diminished psychological well-being (abnormal behaviour, inactivity, vocalisation, self-directed grooming) or for species-typical activities (feeding, playing). The difference in time spent exploring between macaques housed on the lower row and those housed on the upper row approached significance, with lower-row-housed animals spending more time exploring. Although lower-row cages are significantly darker than upper-row cages at our facility, the data from the present study demonstrate that the diminished lighting and other supposed disadvantages experienced by lower-row-housed monkeys have few behavioural consequences. Thus, there are now additional empirical data that suggest that lower-row-housed monkeys are not suffering in a "monkey cave", and that the findings of research projects using two-tiered housing systems are unlikely to be compromised.