SummaryWe sought to test the hypothesis that hyperinsulinemiaper sealters the flux of surface active material (SAM) into tracheal fluid by continuously infusing insulin (0.24 ± 0.04 units/kg/hr, mean ± S.E.) from 112 through 135 days gestation into five chronically catheterised fetal lambs, from which tracheal fluid could be collected.Serum insulin levels in these fetuses (95 ± 10 μU/ml) were greater than in five chronically catheterised control fetuses of the same gestational age (10 ± 1 μU/ml,P< 0.001) and in the mothers (38 ± 6 μU/ml,P< 0.001). Serum glucose levels in the insulin-treated fetuses (10 ± 1 mg/dl) were lower than in the control fetuses (19 ± 1 mg/dl,P< 0.001) and in the mothers (60 ± 3 mg/dl,P< 0.001). Arterial blood gases (pH 7.37 ± 0.01, PO223.3 ± 0.05 mm Hg, PCO241.5 ± 0.9 mm Hg) and hematocrit (33 + 1% at 127 days gestation and 31 ± 1% at 135 days gestation) in the insulin treated fetuses were not different from the controls.SAM flux into the tracheal fluid of the insulin-treated fetuses was 1 μg/kg/hr, coefficient of variation 373%. This was lower than SAM flux in the control fetuses (26 μg/kg/hr, coefficient of variation 28%,P< 0.01). Moreover, among the control fetuses, SAM began to appear in tracheal fluid at 119 days gestation and was present in all five fetuses by 125 days gestation, whereas SAM did not begin to appear in the insulin-treated fetuses until 127 days gestation and did not appear at all in three of them.