To understand better developmental changes in body fluid dynamics, we studied thoracic duct lymph flow in 9 newborn and 5 adult sheep. The experiments were carried out under general anaesthesia following bilateral ligation of the renal vessels and ureters. After a 30 min control period, we administered three successive 5-min intravenous infusions of isotonic saline equivalent to 2% of body weight each, at 30-min intervals. The average basal lymph flow rate was 0.157 +/- 0.033 (SEM) ml.min-1.kg-1 and 0.046 +/- 0.018 ml.min-1.kg-1 in newborns and adults respectively (p less than 0.05). Fluid overloading resulted into similar intravascular retentions at the end of each 30-min period in both groups although the increase in lymph flow was repeatedly more than three times higher in the newborns. The more pronounced lymph flow response in the newborns could not be accounted for only on the basis of a difference in capillary filtration. We speculate that interstitial forces and/or the lymphatic pumping activity play a greater role in facilitating fluid movements in the newborn lamb than in the adult ewe. Overall, the higher capacity of the newborn to eliminate excess interstitial fluid constitutes a significant factor in the body's defense against oedema.