We studied the effect of exogenous applications of GAon3 on rubisco activation in soybean leaves. Activity at 0.1 μM GA3 was significantly greater than in leaves receiving no treatment Rubisco content snowed patterns of change similar to that for activity. These data suggest that activity was associated with the amount of rubisco protein, and that the activation and induction of rubisco was promoted by GA3. The degree of intensity of the 50 and 14.5-kD polypeptides (identified as the large and small subunit, respectively, of rubisco by SDS-PAGE analysis) at 0.1 μM GA3 was significantly higher than that for the control, indicating that GA3 affected both subunits. The stimulation effects of rubisco activation by GA3 seem to be caused by the expression of rubisco genes at the transcriptional level. Assuming that these GA3 effects were related to rubisco activase, we also determined the activity and content of that enzyme. Its activity at 0.1 μM GA3 increased more than did the control. A similar pattern of change was observed for rubisco activase content The intensity of two 46- and 42-kD polypeptide bands at GA3 was higher than for the corresponding bands at the control. Therefore, the change in rubisco activase levels may lead to altered levels of rubisco.