Exposure of rabbits to 15.5% oxygen during pregnancy resulted in a 17% decrease of birth weight and a neonatal mortality of 19% as opposed to 1% in the control groups, just corresponding to the effects obtained previously by moderate exposure of pregnant rabbits to carbon monoxide. Since additive effects of hypoxia exposure and carbon monoxide exposure are supposed to occur, it is concluded that women living at a high altitude should be warned particularly about the risk of delivering small babies if they have been smoking during their pregnancy.