Cells of Aphanocapsa 6714 were subjected to alternating ligh-dark periods (flashing-light experiments). The corresponding activation (in the light) and inactivation (in the dark) of the reductive pentose cycle was measured, in vivo, from initial rates of 14CO2 incorporation and also by changes in the total concentration of 14C and 32P in soluble metabolites. Two principle sites of metabolic regulation were detected: (i) CO2 fixation was inactivated 15 to 20 s after removal of the light source, but reactivated rapidly on reentering the light; (ii) hydrolysis of fructose-1,6-diphosphate (FDP) and sedoheptulose-1,7-diphosphate (SDP) by their respective phosphatase(s) (FDP + SDPase) was rapidly inhibited in the dark but only slowly reactivated in the light. The time required for reactivation of FDP + SDPase, in the light, was on the order of 20 to 30 s. As a consequence of the timing of these inactivation-reactivation reactions, newly fixed CO2 accumulated in the FDP and SDP pools during the flashing-light experiments. Changes in the concentrations of the adenylate pools (mainly in the levels of adenosine 5'-triphosphate and adenosine diphosphate) were fast in comparison to the inactivation-reactivation reactions in the reductive pentose cycle. Thus, these regulatory effects may not be under the control of the adenylates in this organism. The activation of CO2 fixation in the light is at least in part due to activation of phosphoribulokinase, which is required for formation of ribulose-1,5-diphoshate, the carboxylation substrate. Phosphoribulokinase activity in crude extracts was found to be dependent on the presence of strong reducing agents such as dithiothreitol, but not significantly dependent on adenylate levels, although adenosine 5'-triphosphate is a substrate.