In this experiment we investigate the effect of photoperiod on locomotor activity, body mass, food intake, growth efficiency (relationship between body mass change and food intake), energy expenditure, and body composition in growing Wistar rats. Two groups of animals were subjected to either a long, LD 18:6 (n = 8) or short photoperiod, LD 6:18 (n = 7), during a period of 190 days after weaning. Activity, body mass, food intake, and energy expenditure were measured during the study, as well as body composition at the end of the experiment. We show that growing rats exposed to short photoperiod (a) have a lower rate of weight gain, (b) have similar energy intakes, (c) have lower growth efficiency, (d) have lower daily energy expenditure and resting metabolic rate, and (e) gain less lean body mass than those exposed to long photoperiods. We suggest that the distribution of energy expenditure and food intake over the total 24-h cycle may be responsible for the differences in body weight gain between the two photoperiods.