Eucalyptus gomphocephala A.DC seedlings grown horizontally for 103 days had less terminal shoot elongation and higher internal and emanated ethylene levels in the basal portion of the stem than seedlings grown vertically under otherwise identical conditions. Horizontal seedlings had greater radial growth in the upper stem half than in the lower half of the basal portion of the stem. Upper halves contained 60-80 percent tension wood by volume, lower halves 0-10 percent tension Wood. Radial growth in vertical seedlings was symmetrical, and they contained negligible tension wood. Upper halves of the basal portion of the stem of horizontal seedlings had greater amounts of internal and emanated ethylene than lower halves and vertical seedling halves. Ethylene differences between random halves of vertical seedlings were smaller than differences between upper and lower halves of horizontal seedlings. The data suggest an association between increased ethylene levels and tension wood formation. * This work was made possible through the award of a Fulbright Postdoctoral Fellowship and an Honorarium to N. D. Nelson by the Australian-American Educational Foundation and CSIRO Division of Building Research, respectively, and is part of a larger body of research by the authors on hormonal aspects of wood quality in rapidly grown Eucalyptus spp. Appreciation is expressed to Peter Fitzgibbon and Jugo Ilic for competent technical assistance.