Sunflower (Helianthus annuus L.) and tobacco (Nicotiana tabacum L.) were grown in the laboratory and leaves were taken from field-grown birch trees (Betula pendula Roth). Chlorophyll fluorescence, CO2 uptake and O2 evolution were measured and electron transport rates were calculated, J(C) from the CO2 uptake rate considering ribulose-1,5-bisphosphate (RuBP) carboxylation and oxygenation, J(O) from the O2 evolution rate, and J(F) from Chl fluorescence parameters. Mesophyll diffusion resistance, r(md), used for the calculation of J(C), was determined such that the in vivo Rubisco kinetic curve with respect to the carboxylation site CO2 concentration became a rectangular hyperbola with Km(CO2) of 10 microM at 22.5 degrees C. In sunflower, in the absence of external O2, J(O) = 1.07 J(C) when absorbed photon flux density (PAD) was varied, showing that the O2-independent components of the alternative electron flow to acceptors other than CO2 made up 7% of J(C). Under saturating light, J(F), however, was 20-30% faster than J(C), and J(F)-J(C) depended little on CO2 and O2 concentrations. The inter-relationship between J(F)-J(C) and non-photochemical quenching (NPQ) was variable, dependent on the CO2 concentration. We conclude that the relatively fast electron flow J(F)-J(C) appearing at light saturation of photosynthesis contains a minor component coupled with proton translocation, serving for nitrite, oxaloacetate and oxygen reduction, and a major component that is mostly cyclic electron transport around PSII. The rate of the PSII cycle is sufficient to release the excess excitation pressure on PSII significantly. Although the O2-dependent Mehler-type alternative electron flow appeared to be under the detection threshold, its importance is discussed considering the documented enhancement of photosynthesis by oxygen.