SummaryExperiments were carried out on foetal lambsin uterofrom 103 to 133 days' gestation, to measure the daily flow and solute composition of liquid from the foetal lung. The mean daily flow from foetuses with a tracheal fistula was 231 ml/day (9·6 ml/h), while the mean hourly flow in foetuses with a tracheal loop was 5·1 ml/h. The pattern of flow of lung liquid was intermittent and irregular, with short periods of rapid flow interspersed with longer periods of slow and no How. The Na+, Ca2+, Mg2+, Cl−and urea concentrations showed no significant change with increasing gestational age, but the K+concentration showed a significant rise. It is concluded that the lung of the foetal lamb behaves as an exocrine gland, secreting a substantial volume of liquid of a relatively constant composition.