This study describes the effects of prolonged (48 h) infusion of cortisol into ovine foetuses (100-110 days of gestation: term is 150 days) at a time when endogenous plasma cortisol concentrations are < 5 nmol/L. 2. In four chronically cannulated foetuses (107 +/- 0.9 day) the infusion of saline (0.9% NaCl; w:v 0.19 mL/h, 48 h) had no effect on blood pressure, renal function, or composition of amniotic and allantoic fluids. 3. In six foetuses (107 +/- 1 day) the infusion of cortisol (250 micrograms/h) increased plasma cortisol concentrations from 4.1 +/- 0.7 to 118 +/- 9 nmol/L (P < 0.001), increased mean arterial pressure from 34 +/- 1 to 40 +/- 1 mmHg (P < 0.001), increased glomerular filtration rate (P < 0.05), urine flow rate, and free water clearance (P < 0.01). 4. There was a significant increase in excretion rates of potassium and creatinine as a result of cortisol infusion, but no natriuresis, indicating some functional maturation of the fetal kidney. 5. Cortisol infusion had no effect on the volumes of amniotic and allantoic fluids; allantoic fluid composition was unchanged; significant decreases occurred in amniotic fluid osmolality, sodium and chloride concentrations, and in lung liquid osmolality, potassium, creatinine, magnesium, glucose and fructose concentrations. 6. Thus prolonged exposure of the immature ovine foetus to elevated cortisol concentrations produced significant alterations in the water and electrolyte balance of the foetus.
