The in vivo activity of ribulose-1,5-biphosphate carboxylase/oxygenase (Rubisco) is modulated in response to light intensity by carbamylation of the active site and by the binding of sugar phosphate inhibitors such as 2'-carboxyarabinitol-1-phosphate (CA 1P). These changes are influenced by the regulatory protein Rubisco activase, which facilitates the release of sugar phosphates from Rubisco's catalytic site. Activase levels in Nicotiana tabacum were reduced by transformation with an antisense gene directed against the mRNA for Rubisco activase. Activase-deficient plants were photosynthetically impaired, and their Rubisco carbamylation levels declined upon illumination. Such plants needed high CO2 concentrations to sustain reasonable growth rates, but the level of carbamylation was not increased by high CO2. The antisense plants had, on average, approximately twice as much Rubisco as the control plants. The maximum catalytic turnover rate (k cat) of Rubisco decreases in darkened tobacco leaves because of the binding of CA 1P. The dark-to-light increase in k cat that accompanies CA 1P release occurred to similar extents in antisense and control plants, indicating that normal levels of activase were not essential for CA 1P release from Rubisco in the antisense plants. However, CA 1P was released in the antisense plants at less than one-quarter of the rate that it was released in the control plants, indicating a role for activase in accelerating the release of CA 1P.