Maternal treatment with corticosteroids before preterm delivery is effective in reducing the incidence of respiratory distress syndrome and neonatal mortality. We hypothesized that corticosteroids might lower the lung hyaluronan concentration. Twenty-five rabbit dams (term = 31 d) with timed pregnancies were injected s.c. with 0.75 mg of betamethasone or saline (controls) 1 d before delivery. In addition, two dams delivered at 25 d of gestation were injected with 0.75 mg of betamethasone on two consecutive days before delivery. A total of 238 live pups were delivered by preterm cesarean section at 25, 27, 28, or 29 d of gestation and killed immediately. Their lung hyaluronan concentrations were measured with a radiometric assay, and wet/dry lung weight ratios were determined. Lungs of rabbit pups exposed antenatally to betamethasone and delivered at 25 or 27 d of gestation, but not at 28 or 29 d, displayed significantly (p = 0.001 and p = 0.008, respectively) lower hyaluronan concentrations than control pups, accompanied by less intense subepithelial staining for hyaluronan in alveolar walls. There was no significant difference in wet/dry lung weight ratio between pups exposed to one dose of betamethasone and controls. Antenatal corticosteroid exposure lowers the lung hyaluronan concentration in preterm rabbit pups delivered at 25 or 27 d of gestation, but not in those delivered at 28 or 29 d.