Evidence for a cyclic electron flow has been sought by study of the steady-state poise of P700 and rate of photoreaction 1 in three cyanobacteria. Under an actinic light 1 (440 or 680 nm) the rate of photoreaction 1 is limited by the rate of electron supply provided by photoreaction 2 and by all return electron flow from low potential donors such as ferredoxin and NAD(P)H. Plots of p, the steady-state fraction of P700 reduced, versus the reciprocal intensity, 1/I, yield linear segments of slope Ip. From considerations of a simple model the slopes and extrapolated intercepts of the linear segments provide estimates of the rate of return electron flow. Analysis shows that the total return electron flow cannot be large, by one estimate not more than three times the rate of dark respiration. This result leads to a conclusion that cyclic electron flow (and any dependent phosphorylation) is not a significant process in these cyanobacteria at ordinary light intensities.