Differences between sexes in the maturation of the motor activity rhythm have been studied in rats (10 females and 7 males) born and raised under LL and isolated from the day of weaning. Motor activity data were registered every 30 minutes for 45 days after weaning. Periodogram analysis was applied to each animal data and the daily power spectra were calculated on the basis of the period, tau. The process of rhythm evolution was studied by means of an analysis of variance of a multivariant linear model, and by defining two mathematical functions that fit to the evolution of the total daily motor activity (TDMA) and the power content of the circadian harmonic (PCCH) through time: a sigmoidal function, which presents an inflexion was used for the study of the evolution of the PCCH values through time: and a gaussian function, which presents a minimum was used for the evolution of the TDMA values. Results indicate that males show more ultradian components than females just after weaning, and take a longer time to acquire the circadian harmonic as the main harmonic in the daily power spectrum. The acquisition of the circadian character is preceded by a decrease in the TDMA, in the case of the males and is followed by a decrease in TDMA in the case of the females. These results suggest sexual differences in the organization and maturation of the circadian system in rats.