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1. The fluid sacs and bladders of sixteen foetuses in fourteen ewes were catheterized between 81 and 92 days gestational age and the rumens of four ewes were also catheterized.

2. Between 95 and 145 days gestational age in forty-six 24 hr experiments hourly samples of maternal plasma and foetal urine were obtained and in fifteen experiments foetal fluid samples were also taken at 4- to 6-hr intervals.

3. The osmolality, pH, and concentrations of sodium, potassium, chloride, glucose, fructose and urea were measured on all samples.

4. During experiment there was no significant variation in the composition of amniotic or allantoic fluid. Marked changes in osmolality occurred in maternal plasma and foetal urine when ewes drank after feeding, but not in ewes that received water intrarumenally via catheter while feeding or in fasting ewes. Post-prandial changes in maternal plasma osmolality may have altered transplacental water fluxes and as a result foetal plasma volume and osmolality.

5. The results suggest that the foetus alters renal water retention by varying antidiuretic hormone (ADH) secretion in response to changes in blood volume and at later gestational ages plasma osmolality as well.

6. Post-prandial changes in the [Na⁺]/[K⁺] ratio of foetal urine suggested that foetal adrenocorticotrophin (ACTH) secretion is influenced by variations in foetal blood volume and glucose concentrations.

7. The post-prandial changes in foetal urine composition observed here support previous suggestions (Mellor & Slater, 1972) about the role of foetal urine in foetal fluid formation which were based on gestational changes in the composition of foetal fluids and urine sampled once daily during the post-absorptive state.