The length of the cyclic heat periods of virgin albino rats was found to increase from the age of 5 to 7 months to 15 to 16 months. There was no change in the length of the oestrous cycle. From the age of 17 months the duration of each heat period and the number of females displaying heat behaviour were again reduced. Identical physiological changes in the pattern of heat were found in multiparae. Females with a short heat period exhibit a stronger approach tendency towards a sex incentive during heat than the ones in which the heat lasts for several days.