Exchange of CO2 and O2 and chlorophyll fluorescence were measured in the presence of 360 μ1 · 1(-1) CO2 in nitrogen in Helianthus annuss L. leaves which had been preconditioned in the dark or at a photon flux density (PFD) of 24 μmol · m(-2) · s(-1) either in 21 or 0% O2. An initial light-dependent O2 outburst of 6 μmol · m(-2) was measured after aerobic dark incubation. It was attributed to the reduction of electron carriers, predominantly plastoquinone. The maximum initial rate of O2 evolution at PFD 8000 μmol · m(-2) · s(-1) was 170 μmol · m(-2) · s(-2) or about four times the steady CO2-and light-saturated rate of photosynthesis. Fluorescence measurements showed that the rate was still acceptor-limited. Fast O2 evolution ceased after electron carriers were reduced in the dark-adapted leaf, but continued for a short time at the lower rate of 62 μmol · m(-2) · s(-1) in the light-adapted leaf. The data are interpreted to show that enzymes involved in 3-phosphoglycerate reduction are dark-inhibited, but were fully active in low light. In a dark-adapted leaf, respiratory CO2 evolution continued under nitrogen; it was partially inhibited by illumination. Prolonged exposure of a leaf to anaerobic conditions caused reducing equivalents to accumulate. This was shown by a slowly increasing chlorophyll fluorescence yield which indicated the reduction of the PSII acceptor QA in the dark. When the leaf was illuminated, no O2 evolution was detected from short light pulses, although transient O2 production was appreciable during longer light pulses. This indicates that an electron donor (pool size about 2-3 e/PSII reaction center) became reduced in the dark and the first photons were used to oxidise this donor instead of water.