We have examined the effects of changes in functional residual capacity (FRC), determined by positive and negative body surface pressures, on the breathing pattern of intact newborn rats, before and after barbiturate anesthesia. With distending pressures (between 1 and 4 cmH2O) minute ventilation decreased mainly due to a prolongation of the expiratory time. This response was more marked after anesthesia and accompanied by a fall in tidal volume. The time of peak expiratory flow (TE'), an index of expiratory flow resistance, was not changed before anesthesia and only slightly decreased after anesthesia. With collapsing pressures between 1 and 2 cmH2O only small changes in breathing pattern occurred, whereas the TE' increased in all cases and the flow profile indicated a maintenance of lung volume during expiration. These data indicate that tonic vagal information is present in the newborn rat and is substantially enhanced after barbiturates. The result that changes in breathing pattern are not fully matched by the changes in TE' and expiratory flow profile may indicate that the receptors which control the respiratory pattern are not the same as those involved in the regulation of the expiratory flow. The pressure-volume curve of the respiratory system was similar before and after anesthesia, and the intercept was close to the zero pressure value, indicating that the FRC of the newborn rat, differently from the human baby, is not actively maintained above the resting volume of the system.