We have reinvestigated several aspects of the Mehler reaction in isolated chloroplasts. We have confirmed that the rate of oxygen uptake is accelerated by a number of biological and artificial electron carriers. The Mehler reaction also responds to uncoupling agents and exhibits photosynthetic control with ADP. In the presence of catalase, steady-state oxygen exchange may be established and lead to such apparent anomalies as oxygen uptake in the light followed by oxygen evolution in the dark. The steady state is a function of catalase concentration, light intensity, and the presence and concentration of electron carriers, etc., which affect the rate of the Mehler reaction.