Motor activity circadian rhythm of 32 rats belonging to three consecutive generations of rats from a laboratory strain, has been studied. This rhythm has been registered from the day of weaning (at 21 days) to at least 30 days after. The pattern of this rhythm has been analyzed by means of a Fourier analysis, by which the daily power spectrum of each rat was obtained. Based on the mean power spectrum of each animal, the variance among the different families and generations has been calculated. Results show that the variance increases in a statistically significant way with the succeeding generations. It can be suggested that in the third generation, the characteristics of the progenitors are dispersed, confirming the genetic character of the rhythm and suggesting a multigenic character for the transmission of the circadian rhythm of motor activity.