Mongolian gerbils were isolated for one minute on each of the first 14 days of life. The ultrasounds which they produced were displayed on a recorder which allowed measurement of the sound level, duration, shape and temporal patterning of the calls. These components of the pup's response changed with age in various ways. Most of the ultrasounds produced occurred in bouts of regularly spaced calls. The bout length was longest on day 4, and then decreased, but the rate of calling within bouts increased with age. The mean sound level of the calls reached a peak on day 6 and then declined, but one or more pups at each age could produce loud calls. The total rate of production of single calls changed little with age, but production of multiple calls reached a peak on days 4 and 5, and then declined. It seems that the ability of pups to call rapidly and to produce a series of loud, complex calls increases with age. As calling declined, scrabbling movements with the legs increased. Temperature regulation and motor ability are improving during this time. These changes in calling are related to changes in maternal behaviour, especially nest building, which peaked on day 6 in another study. Observations on control litters suggested that daily handling and testing increased the rate of calling on day 5.