Closing capacity (CC; the lung volume at which dependent lung zones cease to ventilate, presumably as a result of airway closure) and standard lung volumes were measured in twenty-four normal women 21 to 29 years of age in the seated and supine positions. Measurements were made serially during pregnancy and once 6 to 8 weeks post partum (the control period). CC was unchanged during pregnancy, while functional residual capacity (FRC) fell by approximately 15%. The difference between FRC and CC (FRC-CC) was reduced during pregnancy, but in one of the subjects when seated, and in only two supine subjects did CC exceed FRC. FRC-CC was reduced in the supine position as compared to seated. The reduced FRC-CC during pregnancy offers a potential explanation for the observations of abnormally low arterial oxygen tensions (Pao2) during pregnancy in some women, and of reduced Pao2 in pregnant women in the supine as compared to upright position.