The superoxide anion radical is known to be the first product of photosynthetic reduction of oxygen mediated by a variety of electron carriers. The effectiveness of many of these electron carriers as herbicides, and the toxicity of the superoxide they produce, have been suggested to rule out oxygen reduction as a physiological component of normal photosynthesis. Here results with isolated spinach chloroplasts are presented that demonstrate that the related catecholamines adrenaline and dopamine mediate photosynthetic reduction of oxygen. Complete inhibition by added superoxide dismutase of light-dependent oxygen uptake by isolated chloroplasts and of the electron transport it supports indicates that superoxide is an obligatory catalytic intermediate, not a product, in adrenaline- and dopamine-mediated oxygen reduction. These compounds might function as chemical analogues of a proposed natural mediator, or oxygen-reducing factor, that allows oxygen reduction to participate in energy transduction in photosynthesis. The identity of the putative natural mediator and the role of oxygen reduction in photosynthesis are discussed. The fully oxidized form of adrenaline, adrenochrome, also acts as a mediator of photosynthetic oxygen uptake, but only by reducing oxygen to superoxide.