Barley seeds have been germinated in gas mixtures containing ethylene (up to 20 vpm) and various amounts of oxygen (0.5–21.0 per cent). When oxygen was adequate, ethylene had no effect on germination but decreased root growth and increased top growth. Ethylene-treated roots were short, broad and curled. When inadequate oxygen slowed seedling growth, ethylene had no effect on roots but increased top growth. Effects of carbon dioxide concentration and of the residual effects of ethylene on seedling growth are also discussed.