1. Several months' measurements of activity under natural illumination conditions near the equator (Colombia) show that changes in night illumination over the lunar cycle influence the activity of nocturnal mammals in various ways. 2. Night monkeys (Aotus trivirgatus) largely limit their activity to dusk and dawn during the new moon period. During the full-moon period they are very active throughout the night (Fig. 5). In the case of the neotropical phyllostomatid batArtibeus lituratus, moonlight has the effect of reducing activity. Accordingly when the moon waxes, this species decreases its activity in the first half of the night, when the moon wanes in the second half of the night, and when there is a full moon throughout the whole night (Fig. 6). African fruit bats (Rousettus aegyptiacus) reach their maximum activity when the moon waxes, the phyllostomatid batPhyllostomus hastatus when it wanes (Figs. 8 and 9). Both show least activity when there is a full moon. In the case of golden hamsters (Mesocricetus auratus) no light-induced changes in activity and activity pattern were observed under natural illumination conditions. 3. The results point to a division into 3 basic types of reaction to lunarinduced changes in the intensity of night illumination (Aotus, Artibeus, andMesocricetus Types), as well as various transitional forms. They suggest that light influences activity not only through the circadian system but also directly, and also that these two components can be developed in a greater or lesser degree, and there are proportional and differential effects of light in both of them. 4. In the case ofRousettus a direct relation between behaviour under natural illumination conditions and the "optimum function" of the dependence of activity on the light intensity can be observed. It is to be expected that this is true of all species in which similar light-dependent activity optima or activity changes under natural illumination conditions have been observed.