Resumen: All animals including primates show regular variations in their behavioural activities and physiological functions. In numerous species diel, ultradian, infradian, annual and/or lunar rhythms have been established; however, the use of the focal sampling method does not provide information on hour - to - hour nor day - to - day variations. In behavioural field studies dealing with primate activity rhythms and their modulation by environmental variables the possible dual, synchronizing and/or masking effect of these variables is often ignored. In the present study, we analyze the relation between astronomical and meteorological parameters and various parameters of the rest - activity rhythm in Ateles geoffroyi, a species scarcely investigated with respect to biological rhythms. These Ateles were kept under semi - natural conditions, exposed to natural light, temperature and humidity cycles, and sky state. Motor activity was recorded with actiwatch accelerometers during 180 days (5 - min intervals), starting from the autumn equinox.