The quantum efficiencies of photosystems I and II (PSI and PSII), [NADP]/[NADPH] ratios, and the activities of chloroplastic fructose- 1,6-bisphosphatase and NADP-malate dehydrogenase were measured in intact pea (Pisum sativum L.) leaves in air following the transition from darkness to 750 microeinsteins per square meter per second irradiance. PSII efficiency declined from a low value to a minimum within the first 10 to 15 seconds of irradiance, after which it increased progressively to the steady-state value. The resistance of electron flow between the photosystems was high at this time, but it was not the principal factor limiting electron flow. Oxidation of P700 was restricted by acceptor side processes for approximately the first 60 seconds of illumination. Once the acceptor side limitation was relieved, the oxidation state of P700 was used to estimate the quantum efficiency of electron transport by PSI. This was observed to increase progressively with time. The quantum efficiencies of both photosystems increased in parallel, consistent with a predominant role for noncyclic electron transport. Fructose-1,6-bisphosphatase activity increased in an approximately sigmoidal fashion with time of irradiance, paralleling the changes in the quantum efficiencies of the photosystems. In contrast, the activation of NADP-malate dehydrogenase did not show a lag period but increased with time, reaching a maximum value at about 50 seconds of illumination, after which it declined. The NADP pool was not extensively reduced during the first 10 seconds of illumination, but became so subsequently. It remained in the reduced state until about 60 seconds of illumination and then became relatively oxidized. The empirical relationship between NADP-malate dehydrogenase activity and the reduction state of the NADP pool supports the suggestion that NADP-malate dehydrogenase activity is a useful estimate of the reduction state of the stroma.