Abstract We studied the effect of different constant light intensities (0.25 lux of dim red light and 4 lux, 40 lux and 500 lux of white light) on the motor activity rhythm of rats. From the day of weaning, rats were kept in individual cages and motor activity was recorded every 15 minutes for 50 days, by a system with two crossed infrared beams. Data were analyzed by means of the periodogram of Sokolove and Bushell and by Fourier analysis. The mean waveform of the rhythm of the adult animals was also calculated and quantified by defining 3 mathematical indices that describe several aspects of the waveform, as noise, stability and abruptness. Under dim red light, the animals show from the beginning of the record a circadian component. In the groups kept under white light, a large number of ultradian components appear during the first days after weaning, and a well defined circadian component appears at about day 10. The waveform that the animals acquire in adulthood depends on the light intensity. The eff...