The number of ultrasonic cells produced per minute by infant mice in response to room temperature increases during the 1st week after birth and decreases during the 2nd. These vocalizations show a distinct bout organization that changes with age. The initial increase in call production is due to an increase in the number of bouts whereas the subsequent decline is primarily due to bouts of fewer calls. The rate of calling within bouts, however, increases steadily with age. These findings are discussed in terms of maturational and motivational changes.