The light activation of photosynthesis has been investigated in spinach palisade cell protoplasts. (1) After a short induction period, maximal rates of photosynthesis are achieved. (2) [14C]Bicarbonate initially labels anionic compounds in the chloroplast and then in the extrachloroplast compartments. These pools saturate within 2-4 min and radioactivity accumulates mainly in sucrose in the extrachloroplast compartment, in starch and in cationic compounds. (3) Enzymic determinations were made of metabolite levels during the induction period in the chloroplast and extrachloroplast compartments. There is no general build-up of intermediates. Perturbations of individual intermediates occurred, consistent with the activation of specific enzymes. (4) It is suggested that fructose-1,6-bisphosphatase and ribulose-1,5-bisphosphate carboxylase may limit flux in the Calvin cycle during induction. (5) The onset of sucrose synthesis is not accompanied by accumulation of intermediates in the cytosol. It is suggested that sucrose phosphate synthase or sucrose phosphate phosphatase is activated. (6) Measurements of metabolites in whole leaves during a 24 h illumination cycle confirmed that substrates are not depleted during the dark period, and that the onset of photosynthesis is not accompanied by a rise in intermediate levels. (7) It is concluded that the causes of the induction lag in protoplasts can differ from those in isolated chloroplasts.