The effects of the light pattern on the evolution of circadian motor activity rhythm of rats were studied in this work. Three different light patterns were used: LL (bright light, 300 lux), DD (dim red light) and LD (12:12 cycles). The animals used for the experiment were born and kept under each condition. At the day of weaning (21-22 days old) animals were isolated and their motor activity was detected by means of an inductive system. Data were recorded every 30 minutes for the first month after weaning. Periodogram analysis was applied to each animal's data and the daily power spectra were calculated on the basis of the endogenous period, tau. The evolution of the rhythm was studied by examining the changes of the whole power spectra of the motor activity function, obtained by means of a Fourier analysis, through time. The power content of the circadian harmonic (PCCH) was considered to be a measure of the circadian character of the function. Results showed the predominancy of ultradian harmonics, when the animals were young, especially in LL, and the increase of the PCCH through time in all cases. Animals under LL show the circadian harmonic to be the main harmonic of the spectra at about day 15 after weaning, while the animals under DD show this harmonic as the main one from the first day. However, the power content of this harmonic increased until day 10. LD animals also showed the first harmonic as the main one from the time of weaning increasing the PCCH until day 7. These results are explained in respect to a multioscillatory system.(ABSTRACT TRUNCATED AT 250 WORDS)