Our aim was to determine the effects of oligohydramnios during the last third of ovine gestation on respiratory function in Iambs during their first postnatal month. To induce oligohydramnios, amniotic and allantoic fluids were drained from pregnant ewes, starting at 109.0 ± 2.3 d of pregnancy (term ≈ 148 d). In 10 lambs born at term, respiratory function was studied four times at weekly intervals; a group of nine lambs from normal pregnancies served as controls. Over the 4-wk study period, treated lambs had significantly higher breathing rates and smaller tidal volumes than controls, although the differences diminished with age. Minute ventilation and O2 consumption were the same in each group, and when related to body weight, both declined with age. Treated lambs were nor-moxemic but were hypercapnic compared with controls for up to 4 wk. Functional residual capacity, measured by helium dilution, was the same in each group and increased with age. Static compliance of the respiratory system was lower in treated lambs up to 4 wk; lung compliances were the same in each group, but chest wall compliance was lower in treated lambs than in controls for 4 w k. Postmortem measurements, at 27-28 d, of pulmonary dry weights, DNA contents, and protein contents suggest that the lungs of treated lambs may have been mildly hypoplastic. We conclude that oligohydramnios causes a decreased chest wall compliance, which leads to rapid, shallow breathing and a mild hypercapnia lasting for at least 4 postnatal wk.