Besides rods the retina of the nocturnal greater bushbaby, genus Otolemur, also contains small cones which, however, do not allow color vision. In order to find out whether these cones might be involved in circadian photoreception in the Garnet's galago Otolemur garnettii we determined the threshold for photic entrainment. Activity recordings revealed a short circadian period of 22.6+/-0.7 h subjected to pronounced long-lasting aftereffects. The animals had a relatively high threshold for photic entrainment at about 3-30 lux. This indicates that the cones and/or other, as yet unidentified photoreceptive retinal cells may be involved in circadian photoreception. The galagos' threshold for photic entrainment also depended on the luminosity during the dark phase of the light dark cycles. Results furthermore showed that in Otolemur aftereffects may play a crucial role for circadian entrainment. Light time luminosities just below the individuals' threshold for photic entrainment strongly inhibited the galagos' locomotor activity and, thus, produced pronounced negative masking effects on their free-running circadian activity rhythm. From this it may be inferred that masking direct effects of light are not induced or mediated via the circadian system, i.e. via the circadian pacemaker in the hypothalamic suprachiasmatic nuclei, but at a higher central nervous integrational stage.