Exposure to high levels of estradiol (E2) and progesterone (P) derived from the fetoplacentomaternal unit during the last trimester of pregnancy may play a crucial role in prenatal lung development and immediate postnatal alveolar fluid clearance (AFC). To measure prenatal alveolar formation and postnatal amiloride-sensitive AFC after pharmacological deprivation of E2 and P in utero, fetuses from five sows received an intramuscular depot injection of the E2 receptor blocker ICI 182.780 (ICI) and the P receptor blocker RTI 3021-022 (RTI) and fetuses of five other sows received a placebo injection (control group) during a laparotomy at 90 d of gestation (term gestation, 115 d). Piglets were delivered by cesarean section on d 114 of gestation. Of 95 live-born piglets, 35 were mechanically ventilated. The airways of the right lower lobe were isolated by a balloon catheter wedged in the bronchus and 5% albumin in 0.9% NaCl with or without 1 mmol/L amiloride was instilled. Amiloride-sensitive AFC was calculated from the protein concentration changes in fluid recovered after 120 min as the percentage of absorbed fluid. Lungs were removed under standardized conditions to perform alveolar counts. Prenatal treatment with ICI and RTI resulted in a significantly lower amiloride-sensitive AFC (median, 31%; min-max, -4-58) than placebo (74%, 18-231). Median alveolar counts per visual field were significantly lower in piglets that were exposed to ICI and RTI (38, 21-78) compared with placebo (56, 32-113). We conclude that prenatal E2 and P deprivation significantly impaired alveolar formation and amiloride-sensitive AFC.