The adaptation of the endogenous rhythm of an organism to external cycles may be critical for the development of physiological processes in which energy is expended. We sought to determine whether growth rate depends on the degree of tuning between the external cycle and the manifestation of the circadian rhythms. To do so, we studied the growth rate and the food intake of mice (seven groups of 20 animals each) kept under symmetric LD cycles with different periods (T) of 21, 22, 23, 24, 25, 26, and 27 h, respectively, for 80 days. The mice were then kept in constant darkness for a further 80 days. Throughout the experiment, motor activity was recorded every 15 min for each mouse by means of an actimeter that used crossed infrared beams. Several variables related to the circadian motor activity rhythm were calculated, and correlated with body weight, food intake, and the efficiency of food for growth (food efficiency) calculated as: 100 x body weight increase/ amount of food intake. Results show that these three variables seem to be influenced by the number of circadian cycles that the animal has experienced, but also, and more significantly, by the degree to which the alpha phase of the individual rhythm and the dark phase of the external cycle coincide. Therefore, circadian rhythms would seem to affect the physiological processes that regulate growth and energy expenditure.