In whole plants and intact chloroplasts, photosynthesis does not reach its full rate immediately upon illumination but only after a lag which is believed to reflect an autocatalytic increase in the concentration of carbon cycle intermediates. Autocatalysis has now been observed in a reconstituted system containing envelope-free chloroplasts augmented with ferredoxin and other stromal proteins but only catalytic amounts of ATP and NADP. With ribose 5-phosphate as substrate, the CO(2) dependent O(2) evolution recorded for such mixtures implies rates of "endogenous" or ferredoxin-dependent photophosphorylation as high as 360 mumoles of orthophosphate esterified mg(-1) chlorophyll hr(-1).