To assess whether there is a maturation of the response to tube breathing during the first 10 days of life, we have measured the dynamic ventilatory responses of nine term infants (34 studies) to added tubes equivalent to two anatomical dead spaces. Such a method allows an assessment of the baby's ability to increase dead space ventilation in response to an added stress. There was a significant improvement with age over the first 10 days in the babies' ability to reach "expected" values of minute ventilation when the tubes were added (P = 0.003). The rates at which 63% and maximum adaptation were achieved also increased significantly with age (P less than 0.005). The rate to 63% adaptation appeared to be faster in active compared with quiet sleep (P less than 0.05). The dynamic responses to tube breathing improve over the first 10 days of life. This is likely to be due to a maturation of peripheral chemoreceptor control, although improving lung mechanics may also be important.