The regulation of electron transport between photosystems II and I was investigated in the plant Silene dioica L. by means of measurement of the kinetics of reduction of P700 following a light-to-dark transition. It was found that, in this species, the rate constant for P700 reduction is sensitive to light intensity and to the availability of CO2. The results indicated that at 25 °C the rate of electron transport is down-regulated by approximately 40–50% relative to the maximum rate achievable in saturating CO2 and that this down-regulation can be explained by regulation of the electron transport chain itself. Measurements of the temperature sensitivity of this rate constant indicated that there is a switch in the rate-limiting step that controls electron transport at around 20 °C: at higher temperatures, CO2 availability is limiting; at lower temperatures some other process regulates electron transport, possibly a diffusion step within the electron transport chain itself. Regulation of electron transport also occurred in response to drought stress and sucrose feeding. Measurements of non-photochemical quenching of chlorophyll fluorescence did not support the idea that electron transport is regulated by the pH gradient across the thylakoid membrane, and the possibility is discussed that the redox potential of a stromal component may regulate electron transport.