The processes of activation and deactivation of ribulose‐1,5‐bisphosphate carboxylase purified from wheat have been investigated. Two forms of the enzyme are indistinguishable in terms of ribulose‐1,5‐bisphosphate carboxylation and oxidation but exhibit different rates of activation. One form is slowly activated in saturating CO₂ and Mg²⁺ at moderate temperatures (t_0.5∼ 120 min at 25°C), the other form rapidly activated (t_0.5∼ 8 s). In the presence of the effectors 6‐phosphogluconate or NADPH, significantly lower concentrations of the activating co‐factors can achieve full activation of both enzyme species. However, with another effector, fructose 1,6‐bisphosphate, for the slowly activating species the mode of action is the same as with 6‐phosphogluconate or NADPH, whereas the activation of the rapidly activating species is significantly inhibited. The substrate, ribulose 1,5‐bisphosphate, also inhibits this rapid activation process. A mechanism is proposed for the reactions involving activation that accounts for the differential rates of activation and the response to effectors.