To assess the cardiopulmonary baroreflex in the immature animal, effects of volume expansion on changes in right atrial pressure, renal sympathetic nerve activity, and renal function were measured in chronically instrumented newborn (4-8 days; n = 13) and older lambs (4-5 wk; n = 14). Studies were carried out for 30 min before and 2 h after volume expansion with 6% Dextran 70 (25 ml/kg). Right atrial pressure increased by 4.0 +/- 0.5 mmHg in newborns and by 8.8 +/- 0.6 mmHg in older lambs within 15 min of volume expansion (P less than 0.001). After volume expansion, heart rate decreased in newborns from 237 +/- 6 beats/min to a nadir of 211 +/- 7 beats/min 2 h later (P less than 0.001) but remained constant at control levels of 148 +/- 9 beats/min in older lambs. Maximal inhibition of renal sympathetic nerve activity was achieved at 15 min in older lambs (-50.1 +/- 7.5%) and at 60 min (-58.3 +/- 10.9%) in newborns in which there was a prolonged sympathoinhibition (P less than 0.001). There was also a significant diuretic response in both groups but a limited natriuretic response to volume expansion in newborns (P greater than 0.05) compared with older lambs (P less than 0.001). These data demonstrate the presence of the cardiopulmonary baroreflex in the first week of life in lambs. The sustained sympathoinhibition and bradycardia seen in response to volume expansion in newborns but not in older lambs support the hypothesis that the reflexes controlling arterial pressure and blood volume change with postnatal maturation.