Chlorophyll fluorescence emission at 680 nm (F680) and the rate of CO 2 fixation were measured simultaneously in sections along the length of wheat and maize leaves. These leaves possess a basal meristem and show a gradation in development towards the leaf tip. The redox state of the primary electron acceptor, Q, of photosystern II was estimated using a non-invasive method. Distal mature leaf sections displayed typical F680 induction curves which were generally anti-parallel with CO 2 fixation and during which Q became gradually oxidised. In leaf-base sections net assimilation of CO2 was not detectable, F680 quenched slowly and monotonously without displaying any of the oscillations typical of mature tissue and Q remained relatively reduced. Sections cut from mid-regions of the leaf showed intermediate characteristics. There were no major differences between the wheat and maize leaf in the parameters measured. The results support the hypothesis that generation of the transthylakoid proton gradient and associated ATP production is not a major limitation to photosynthesis during leaf development in either C 3 or C 4 plants. Removal of CO 2 from the mature leaf sections caused little change in steady-state F680 and produced about 50% reduction of Q. When O2 was then removed, F680 rose sharply and Q became almost totally reduced. In immature tissue unable to assimilate CO2, removal of 0 2 alone caused a similar large rise in F680 and reduction of Q whilst removal of COg had negligible effects on F680 and the redox state of Q. It is concluded that in leaf tissue unable to assimilate CO> either because CO2 is absent or the Abbreviations: F680=chlorophyll fluorescence emission at 680 nm; (PSI=photosystem I; PSII=photosystem II); Q= PSII primary electron acceptor; ApH=transthylakoid proton gradient tissue is immature, 0 2 acts as an electron acceptor and maintains Q in a partially oxidised state. The important implication that 0 2 may have a role in the prevention of photoinhibition of the photochemical apparatus in the developing leaf is discussed.