We quantified the photosynthetic cyclic electron flux (CEF) around Photosystem I as the difference between the total electron flux through PS I (ETR1) and the linear electron flux through both photosystems. Both measurements were made in the whole tissue of spinach leaf discs illuminated in the same geometry and in CO2-enriched air to suppress photorespiration. (1) CEF was negligibly small below 300 umol photons m−2s−1. Above this irradiance, CEF increased approximately linearly up to the highest irradiance used (1,900 umol photons m−2s−1). (2) CEF at a fixed irradiance of 980 μmol m−2s−1 increased by a factor of almost 3 as the temperature was increased from 5 °C to 40 °C. It did not decline, even when the linear electron flux decreased at high temperatures. (3) Antimycin A, at a high concentration, decreased CEF to about 10% of the control value without affecting the linear electron flux. This method appears to be reliable for quantifying CEF non-intrusively. By contrast, estimation of the linear electron flux from chlorophyll fluorescence over-estimated CEF in the above treatments.