Chlorophyll fluoroscence and oxygen evolution have been measured in suspensions of protoplasts isolated from barely leaves. Like the parent tissue, the protoplasts exhibit transients in both fluorescence and the rate of oxygen evolution upon illumination and before a steady state rate of photosynthesis is reached. The two signals are anti-parallel and slightly phase shifted with fluorescence changes preceeding alternation in the rate of oxygen evolution by 10-15s. Fast (t$\approx $15s) and a slow (t$\approx $2 min) fluoroscence peaks were observed, the latter strongly resembling the oscillations described in leaves(Walker, Horton Sivak & Quick, Photobiochem. Photobiophys. 5, 35 (1983)). The amplitude of the oscillation was most pronounced in strong light, and at high(more than 8.0) pH whereas the period was temperature dependent. Antimycin A was found to suppress the slow oscillation but to exaggerate the fast transient, whereas low (<0.1 $\mu $M) DCMU had the reverse effect. These results are discussed in terms of a proposed mechanism to account for these transients and it is suggested that cyclic electron flow may have an important role.