Jackson, M. B., Fenning, T. M., and Jenkins, W. 1985 Aerenchyma (gas-space) formation in adventitious roots of rice (Oryza sativa L.) is not controlled by ethylene or small partial pressures of oxygen.—J. exp. Bot. 36: 1566–1572. The extent of gas-filled voids (aerenchyma) within the cortex of adventitious roots of vegetative rice plants (Oryza sativa L. cv. RB3) was estimated microscopically from transverse sections with the aid of a computer-linked digitizer drawing board. Gas-space was detectable in 1-d-old tissue and increased in extent with age. After 7 d, approximately 70% of the cortex had degenerated to form aerenchyma. The extent of the voids in 1-4-d-old tissue was not increased by stagnant, poorly-aerated external environments characterized by sub-ambient oxygen partial pressures and accumulations of carbon dioxide and ethylene. Treatment with small oxygen partial pressures, or with carbon dioxide or ethylene applied in vigorously stirred nutrient solution also failed to promote the formation of cortical gas-space. Furthermore, ethylene production by rice roots was slowed by small oxygen partial pressures typical of stagnant conditions. Silver nitrate, an inhibitor of ethylene action, did not retard gas-space formation; similarly when endogenous ethylene production was inhibited by the application of aminoethoxyvinylglycine (A VG), aerenchyma development continued unabated. Cobalt chloride, another presumed inhibitor of ethylene biosynthesis, did not impair formation of the gas in rice roots nor did it decrease the extent of aerenchyma even if A VG was supplied simultaneously. These results contrast with those obtained earlier using roots of Zea mays L. We conclude that in rice, aerenchyma forms speedily even in well-aerated environments as an integral part of ordinary root development There seems to be little or no requirement for ethylene as a stimulus in stagnant root-environments where aerenchyma is likely to increase the probability of survival.