Abstract 1. 1. Carbon dioxide emission ( VCO 2) has been continuously recorded in three laboratory animal species (Sprague-Dawley rats, Japanese quail, Hartley guinea-pigs) which differ by their nocturnal and diurnal activities. A 100 lux stimulus has been delivered at various time intervals. 2. 2. A regular alternation of 12, 3 or 1.5 hr light (L) and darkness (D) gives VCO 2 circadian and ultradian rhythms of 24, 6 or 3 hr periods, respectively, in quail and rats. 3. 3. Such circadian and ultradian LD rhythms are not induced in all guinea-pigs. 4. 4. The amplitudes of the VCO 2 responses are greatest at D→L when the animals have a maximum diurnal activity and at L→D when their maximum activity is nocturnal. 5. 5. Interactions between circadian and ultradian rhythms are seen in all LD experiments, as well as in continuous light (LL) or continuous dark (DD). 6. 6. No more well-marked or even inverted VCO 2 responses to the light stimuli may occur after several days of exposure to these LD alternations.