OBJECTIVE: We hypothesized that maternal treatments with betamethasone acetate induce fetal lung maturation comparably to the betamethasone phosphate + betamethasone acetate used clinically. STUDY DESIGN: Ewes with singleton pregnancies were treated with single doses of 0.25-mg/kg or 0.5-mg/kg betamethasone acetate, 4 doses of 0.25-mg/kg betamethasone phosphate, a single dose of 0.5-mg/kg betamethasone acetate + 0.25-mg/kg betamethasone phosphate, 2 doses of 0.25-mg/kg betamethasone acetate + 0.25-mg betamethasone phosphate or vehicle beginning 48 hours before preterm delivery. Fetal lung maturation was evaluated. RESULTS: All treatments induced lung maturation relative to vehicle controls. The relatively insoluble betamethasone acetate resulted in low maternal blood betamethasone and no detectable fetal blood betamethasone in 2 of 3 fetuses, but it induced fetal lung maturation comparable to the 2-dose betamethasone acetate + betamethasone phosphate or 4 doses of betamethasone phosphate. CONCLUSION: A single maternal dose of betamethasone acetate effectively induces fetal lung maturation in sheep with minimal fetal exposure.