Etiolated rice seedlings (Oryza sativa L.) exhibited marked morphological differences when grown in sealed containers or in containers through which air was passed continuously. Enhancement of coleoptile and mesocotyl growth and inhibition of leaf and root growth in the sealed containers (“enclosure syndrome”) were accompanied by accumulation of CO2 and C2H4 in and depletion of O2 from the atmosphere. Ethylene (1 μl 1−1), high levels of CO2, and reduced levels of O2 contributed equally to the increase in coleoptile and mesocotyl growth. The effect of enclosure could be mimicked by passing a gas mixture of 3% O2, 82% N2, 15% CO2 (all v/v), and 1 μl l−1) C2H4 through the vials containing the etiolated seedlings. The effects of high CO2 and low O2 concentrations were not mediated through increased C2H4 production. The enclosure syndrome was also observed in rice seedlings grown under water either in darkness or in light. The length of the rice coleoptile was positively correlated with the depth of planting in water-saturated vermiculite. The length of coleoptiles of wheat, barley, and oats was not affected by the depth of planting. In rice, the length of coleoptile was determined by the levels of O2, CO2, and ethylene, rather than by light. This regulatory mechanism allows rice seedlings to grow out of shallow water in which the concentration of O2 is limiting.