The activity of corn phosphoglycolate phosphatase (EC 3.1.3.18), a bundle sheath chloroplastic enzyme, is modulated, in vitro, both by NADP(H) and adenylate energy charge. The Vmax of the enzyme is increased by NADP (25%) and NADPH (16%) whatever the pH used, 7.0 or 7.9 respective pH of the stroma in the dark and in the light. At both pH, the adenylate energy charge alone has a positive effect with two peaks of activation, characteristics for this enzyme, at 0.2 and a maximum at 0.8 accentuated under nonsaturating concentration of phosphoglycolate. At low energy charge, NADP(H) increased the activation with an additive effect most particularly observed at pH 7.9 under saturating phosphoglycolate concentration; at high energy charge, NADP(H) had a positive or negative effect on the activation, depending on the pH value and the concentrations of substrate and NADP(H).The ferredoxin-thioredoxin system does not regulate the activity since i) DTT addition do not have any effect, ii) the light-reconstituted system containing ferredoxin, ferredoxin-thioredoxin reductase, thioredoxins and thylakoids is not effective either. However, light-dark experiments indicate that phosphophycolate phosphatase can be subjected to a fine tuning of its activity.All these data suggest that light cannot induce a modification of the protein but could exert a tight control of its activity by the intermediate of Mg(2+) and substrate concentrations and the levels of metabolites such as NADP(H), ATP, ADP, AMP. So, the regulation of the activity shown, in vitro, by energy charge and NADP(H) might be of physiological significance.