Respiratory distress syndrome and associated pulmonary surfactant deficiency are more common in male neonates. Androgens have been shown to depress surfactant production. We tested the hypothesis that müllerian inhibiting substance, another fetal testicular product, might inhibit lung maturation measured as disaturated phosphatidylcholine accumulation. Initially, female fetal rat lungs were incubated with fetal testis or ovary or nanomolar concentrations of bovine müllerian inhibiting substance. Cultured lungs produced less disaturated phosphatidylcholine after incubation for 5 days with either testis (p = 0.012) or müllerian inhibiting substance than after coculture with ovary. In more comprehensive experiments, female lung fragments of 17.5 days' gestation, when incubated with nanomolar concentrations of bovine müllerian inhibiting substance or picomolar concentrations of human recombinant müllerian inhibiting substance, showed significant suppression of disaturated phosphatidylcholine accumulation (p less than 0.004) when compared with incubation with vehicle buffer. The suppression of surfactant accumulation produced in vitro by müllerian inhibiting substance, a potent male-specific fetal regressor, may be a factor in the increased susceptibility of male infants to respiratory distress syndrome.