It is shown that the amount of locomotor activity of night monkeys (Aotus trivirgatus) depends very markedly on the intensity of illumination in the darkness phase of an artificial light/darkness cycle 12:12 h. Maximum activity occurs with the degree of brightness of full moon at approximately 0.1 lx. Lesser as well as higher light intensities have an inhibiting effect on the activities of Aotus. These findings are in accordance with the results of activity measurements on the Aotus under natural illumination conditions in the vicinity of the equator.