Submergence in water greatly stimulates internodal elongation in excised stem sections of deep-water rice (Oryza sativa L. cv. "Habiganj Aman II") and inhibits growth of leaf blades and leaf sheaths. The highest rates of internodal growth have been observed in continuous light. Very little growth occurs in submerged sections kept in darkness or incubated under N 2 in the light. The effect of submergence on the growth of deep-water rice is, at least in part, mediated by CzH4, which accumulates in the air spaces of submerged sections. This accumulation results from increased CgH 4 synthesis in the internodes of submerged sections and reduced diffusion of C2H 4 from the tissue into the water. Increased C2H 4 levels accelerate internodal elongation and inhibit the growth of leaves. Compounds capable of changing the rate of C2H 4 synthesis, namely aminoethoxyvinylglycine, an inhibitor of Call 4 synthesis, and 1-aminocyclopropane-l-carboxylic acid, the immediate precursor of CzH4, have opposite effects on growth of internodes and leaves. The enhancement of internodal elongation by C2H 4 is particularly pronounced in an atmosphere of high CO 2 and low 0 2 . The increase in C2H 4 synthesis in internodes of submerged sections is primarily triggered by reduced atmospheric concentrations of 0 2 . The rate of C2H 4 evolution by internodes isolated from stem sections and incubated in an atmosphere of low 0 2 is up to four times greater than that of isolated internodes incubated in air. In contrast, CgH 4 evolution from the leaves is reduced under hypoxic conditions. The effect of submergence on growth of stem sections of deep-water rice can be mimicked by exposing non-submerged sections to a gas mixture which is similar to the gaseous atmosphere in the internodal lacunae of submerged sections, Abbreviations: ACC = 1-aminocyclopropane-1-carboxylic acid; AVG = aminoethoxyvinylglycine namely 3% 02, 6% CO2, 91% N 2 (by vol.) and i gl 1-1 CzH 4. Our results indicate that growth responses obtained with isolated rice stem sections are similar to those of intact deep-water rice plants.