Three quantities, circadian period, amount of activity, and ratio of activity time to rest time of circadian locomotor rhythms in the night monkey (Aotus trivirgatus) were measured under the influence of two constant light intensities. When light intensity was increased from < 0.1 to 100 lx, the circadian period increased, showing that Aschoff’s rule can be valid for this dark active primate species. The ratio of activity time to rest time was shorter under 100 lx than under < 0.1 lx of continuous light, but a consistent change in the amount of activity did not occur between both light intensitie