Cortisol was infused, intravenously, for 4 h continuously into 5 chronically cannulated ovine fetuses at 111-120 days of gestation (term is 142-152 days). The dose used was 100 micrograms/h, and raised fetal blood cortisol concentrations from 8.2 +/- 4.0 to 56.5 +/- 19.0 nmol/l (values are mean +/- SEM). The effects observed were a 4-5 fold increase in sodium and chloride excretion, a doubling of potassium excretion and free water clearance, no significant changes in urine pH, urea and creatinine excretions, and an increase in urine osmolality from 129 +/- 7.5 to 154.4 +/- 11.3 mosmol/kg water. There were no significant changes in any of the measured parameters in 5 fetuses infused with 0.9% NaCl for 4h. It is suggested that the hyponatremia and inability to retain sodium observed in many premature or very low birth weight babies may be due to the fact that their kidneys are behaving as fetal rather than neonatal organs and responding to the high plasma cortisol concentrations found in such babies with a natriuresis.