1. To determine the importance of the kidneys in maintaining the normal volume and composition of foetal body fluids, measurements were made in 11 chronically catheterized foetuses (123–136 days) that had been bilaterally nephrectomized at least 5 days previously and compared with 10 intact foetuses (121–133 days).
2. The nephrectomized foetuses had reduced extracellular (ECV), blood, plasma and interstitial volumes per kg foetal weight (P < 0.005), reduced plasma chloride levels (P < 0.001) and were acidaemic, hypoxaemic and hypercapnaemic (P < 0.05) compared with intact foetuses. They also had reduced lung liquid production (P < 0.05) and reduced lung liquid sodium and osmolality levels (P < 0.05). Their arterial pressure was more variable between foetuses (P < 0.005) and was directly related to ECV/kg (P = 0.013).
3. To determine which of these changes were due to absence of the foetal renin–angiotensin system, seven chronically catheterized nephrectomized foetal sheep (124–132 days) were infused with replacement doses of angiotensin (Ang)II (1.5 μg/kg per h) for 3 days. Six nephrectomized foetuses were infused with 0.15 mol/L saline.
4. The AngII infusion was non-pressor. It prevented the fall in ECV that occurred in the control group (P < 0.05) and foetal plasma chloride concentration rose (P < 0.05). Blood gas status and lung liquid production rate did not change, but lung liquid sodium concentration fell (P < 0.05) and potassium concentration rose (P < 0.05).
5. Bolus injections of AngII (0.3–5 μg) were given to assess vascular sensitivity to AngII. This was not altered by either nephrectomy or AngII replacement.
6. It is concluded that the foetal kidneys are important for the maintenance of the normal volume and composition of foetal body fluids. Angiotensin II, perhaps because it promotes fluid transfer across the placenta, helps maintain foetal ECV and plasma chloride levels.