Significance Spaceflight affects circadian rhythms. It produces a misalignment between the internal body rhythms which may be linked to reduced performance of simple tasks and psychological and sleep disorders. Most people treated for such conditions are female. Objectives This project examines the effects of altered lighting and gravitational environments on the circadian rhythms of body temperature, activity, drinking and vaious performance parameters (such as hand-eye coordination and short term memory) of male and female rhesus monkeys. Results We completed three studies using male rhesus. In the first, data were collected under a light-dark cycle (LD) then under constant light (LL). The period of the performance rhythm was significantly shorter in LL than in LD. There was no significant difference in the mean number of trials per day between LD and LL, but the amplitude of the rhythm was significantly less in LL. In the second study the light-dark cycle was shifted to simulate the effects of jet-lag. Such a phase shift produces a brief desynchronization between the internal rhythms of the body. Drinking and performance responded similarly to advances and delays of the LD cycle. The third study was conducted at 2G on a large diameter centrifuge. Body temperature showed a decrease in circadian amplitude during the first week at 2G, while the timing of the body temperature rhythm was delayed by over 1.5 hours. There were individual differences in performance at 2G; these ranged from noeeffect to a decrease of 50%. Future Directions We will repeat the Phase Shift and 2G studies using female rhesus as subjects, to compare responses between genders. We will continue to examine both mean levels and rhythms in performance in order to determine if the responses parallel those of other body rhythms.