Two-week-old rats were found to emit very little ultrasound in their cage except during arrivals and departures of the mother, when average peak rates of approximately 1 ultrasonic pulse/rat/min were detected. However, when all pups except one were removed from the home cage, the remaining isolated pup emitted ultrasound at a mean rate of 12 pulses/min for at least 30 min. When young rats of this age were placed alone in an unfamiliar test area, the ultrasound pulse rate was approximately 25/min, whereas groups of 4 littermates in the same situation emitted only occasional ultrasonic pulses. If an isolated pup in the novel environment was allowed access to a single anesthetized littermate or mother this also significantly reduced the rate of ultrasound emission, whereas a warm clay model did not.