The monkey mother-infant separation paradigm is one of the best available animal models of grief and loss related depression. Because of the evidence implicating a relationship between disturbances in circadian rhythms and affective behavioral disorders, we studied circadian rhythms of temperature and heart rate in 10 group living infant pigtailed (M. nemestrina) monkeys during the affective behavioral reaction which accompanied 10 days separation from the mother. Best fit (least-squares method) cosine curves were computed for a total of 176 24-h periods for heart rate (HR) and 139 24-h periods for body temperature (BT), providing estimates of the fitted cosine level (approximating the mean value of the data), amplitude (half the value from the peak to the trough as the fitted cosine), and acrophase (time of fitted cosine peak). Significant decreases in cosine level, and delay in acrophase of 1-1.5 h, were found for both body temperature and heart rate during separation. Following reunion with the mother the acrophase occurred progressively later on subsequent days, suggesting possible free running rhythms. These data, in addition to reinforcing the relationship between circadian rhythm regulation and affective behavior, demonstrate that circadian phase shifts can be precipitated by disruption of an attachment bond without changing other circadian synchronizers, and support the view that disturbances in the regulation of circadian systems are one component of the reaction to loss or separation.