SUMMARY1. Catheters were inserted into the maternal and foetal vasculatures of ten ewes, 100-139 days pregnant, and daily samples ofuterine and umbilical blood and maternal jugular vein blood were taken for periods of 5-27 days after operation.2. Catheters were inserted into the fluid sacs of nineteen foetuses, 60-97 days post-conception, and daily samples were withdrawn for up to 90 days from amniotic sacs (eleven foetuses) and for up to 70 days from allantoic sacs (eight foetuses). Maternal jugular plasma was obtained 3 times weekly and an approximation from its composition to that of uterine and umbilical plasma was made using results from the ewes and foetuses with vascular catheters.3. The pH, osmolality, [Na+] 6. Changes during recovery from operation were followed and indicated that maternal and foetal plasma required about 3 days and the foetal fluids up to 7 days before stability of composition was achieved.7. After recovery from operation, daily changes in the composition of each foetal fluid showed the same general pattern in all foetuses, but the absolute values of constituents sometimes showed large differences. D. J. MELLOR AND J. S. SLATER 8. It is suggested that flow of foetal urine into the amniotic sac increased from 80 days gestational age, that urine flow into the allantoic sac decreased until about 100 days but did not cease thereafter, and that relative to foetal urine the influence of foetal pulmonary fluid on amniotic fluid composition was not great.9. A relative impermeability of the amnion appeared to be a major factor influencing amniotic fluid composition, whereas pumping mechanisms in the chorioallantois seem to have been responsible largely for changes in the composition of allantoic fluid.10. The quantity of solute relative to that of water within each sac appears to be a major determinant of changes in foetal fluid volumes.11. Changes in the [Na+] and [K+] of allantoic fluid during the normal course of pregnancy were consistent with an increasing action of mineralocorticoids on pumping mechanisms in the chorioallantois. Similar but more rapid changes seemed to be associated with acute and chronic episodes of maternal hypoglyeaemia. Under these circumstances foetal hypoglycaemia may effect a relative increase in the secretion of foetal corticosteroids having an action on the chorioallantois.12. The results from this study demonstrate clearly the value of using chronically catheterized animals, and it is suggested that their use in physiological studies on the conceptus must eventually supersede that of acute, anaesthetized preparations.

SUMMARY1. The fluid sacs and bladders of ten foetuses and the allantoic sacs of five foetuses were catheterized between 79 and 96 days gestational age and daily samples were withdrawn until lambs were born naturally at 147 days. Maternal jugular plasma obtained daily allowed the nutritional status of each ewe to be regulated and monitored. All lambs were observed for 7 weeks, and at post-mortem no abnormalities were seen in those operated upon in utero. 6. Changes in urine composition suggested large daily variations in the secretion of foetal antidiuretic hormone and also a rapid increase in its secretion during the last 7 days, and particularly the last 2-4 days before birth. 9. Further evidence has been presented supporting the hypothesis that maternal induced foetal hypoglycaemia effects a relative increase in the secretion of foetal corticosteroids having an action on the chorioallantois. Also, high concentrations of maternal plasma corticosteroids may decrease the permeability of the placenta to glucose.

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SUMMARY1. 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-br 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.