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The aim of this study was to characterize the circadian motor rhythm in the spider monkey, A. goeffroyi, in both entrained and free-running conditions. Four monkeys, 2 males and 2 females, were maintained for 32 days in constant light (LL) and 32 days in 13.25:10.75 LD. Before each experimental cycle, and between the LL and LD recordings, animals were maintained on 12:12 LD in their natural outdoor housing with the rest of their group. Each animal had its own Actiwatch collar for recording motor activity. The duration of the activity and rest periods and onset and offset of activity were averaged for each animal and in each condition. Recordings were analyzed with the ACTIWATCH program, and time series were analyzed with PGRAPH periodogram analyses. Actograms, histogram plots and periodograms were created for every subject. The data were also submitted to Fourier analysis using POTENCOR to determine circadian time and peaks under each experimental condition. Finally, Student's t-tests for paired samples were used to determine the differences between the artificial LL and natural LD conditions of every subject. A clear biphasic circadian pattern in the activity?–?rest cycle was observed in both conditions, confirmed by Fourier analysis with a periodicity of 24?h 01?min in LL. In LL the activity showed a dispersed temporal pattern with several small intermingled peaks, while the bimodal pattern of activity was more pronounced in LD with peaks at 08:11?h and 14:45?h. This data demonstrate the presence of a pacemaker with intrinsic ability to oscillate when the organism is under constant external conditions LL, that becomes entrained by the solar day when the animal is exposed to natural LD conditions. [ABSTRACT FROM AUTHOR]