The earliest biochemical marker of ¯oral evocation in the shoot apex of S. oleracea is the doubling of the rate of glucose-6-phosphate dehydrogenase (G6PD) activity 12±15 h after transfer of 4-week-old plants from short days to continuous light i.e. 1±2 h after the leaves are raised to the ¯oral state. Quantitative cytochemical analysis of G6PD activity in the vegetative apices showed that addition of 10ÿ7 M Ca2‡ to the cytochemical enzyme reaction medium for G6PD activity raises the rate of enzyme activity to that seen in the induced apices. Higher concentrations of Ca2‡ result in G6PD inhibition in the vegetative apices and any added Ca2‡ at concentrations of 10ÿ7 M or higher inhibit the G6PD activity seen in both the induced apices and leaf primordia of both types of apex. The addition of EGTA abolishes the cytochemical reaction. The ability of the Ca2‡ to activate the G6PD activity in addition to the incubation medium occurs during the periods of 8±11 h of continuous light, but is already lost by 12 h when no change is achieved by Ca2‡ treatment. This can be interpreted as indicating a point in time close to the moment of ¯oral evocation. A model is proposed in which Ca2‡ is able to activate the inactivated-G6PD molecules in the vegetative apex through increased Ca2‡ ¯ux possibly through the action of plasmalemmal Ca2‡-ATPase activity as part of the ¯oral evocation process.