Daily patterns of food intake and activity were determined for normal cats and cats with pontile lesions. Food intake in the dark and in the light of LD (light/dark) cycles were determined separately by weighing the food, and a 'percentage nocturnal' score was calculated. The measure of activity was infrared photobeam interruptions, with the photobeam placed in front of the cages, over the food bowl. No difference between normal cats and cats with pontile lesions were detected for any of the measures. Food intake of cats was influenced by simulated starlight and moonlight conditions and by the presence of humans. Cats in isolation from humans and human noises exhibited random patterns of activity in constant light and free-running circadian rhythms in constant dark. Idiosyncratic differences in entrainment to LD cycles were found among cats, and the relevance of this variability is noted for studies of photoperiodic phenomena in this species.