To determine the effects of circulating noradrenaline on fetal renal function noradrenaline was infused intravenously into 7 chronically catheterised fetal sheep (127-138 days) at a dose (1 microgram/kg/min) which resulted in plasma levels similar to those which occur during hypoxia. Fetal mean arterial pressure increased by approximately 14 mmHg (p < 0.001) and haematocrit rose (p < 0.005). Glomerular filtration rate rose from 3.85 +/- 0.47 (SEM) to 4.70 +/- 0.50 ml/min (p < 0.05) during the first hour and fractional reabsorption of sodium by the proximal tubule fell (p < 0.05) during the second hour. Urine flow rate increased from 0.61 +/- 0.13 to 1.18 +/- 0.24 ml/min (p < 0.001) and osmolar excretion increased from 78 +/- 15 to 153 +/- 36 mu osm/min (p < 0.005). By contrast lung liquid flow fell (p < 0.05), but the increase in urine flow was much greater than the decline in lung liquid. These findings suggest that during hypoxia, noradrenaline may play an important role in the maintenance of urine flow and consequently amniotic fluid volume and, as suggested by others, in the distribution of fluid between the vascular and interstitial compartments.