The photosynthetic evolution of oxygen by isolated chloroplasts of Spinacia oleracea L. was studied using a modulated oxygen electrode. The enhancement effect, measured as the increase in the relative quantum yield of 650-nanometer light due to the presence of 710-nanometer light, was profoundly influenced by the concentration of inorganic cations in the bathing medium. Chloroplast fragments immersed in a solution containing a very low concentration of MgCl(2) or KCl, did not display enhancement but could be made to do so by raising the concentration of MgCl(2) to 3 mm, or that of KCl to 35 mm. This change in the enhancement properties was completely reversible. The maximum value of enhancement in a MgCl(2) solution appeared to occur at a concentration between 15 and 30 mm, while in KCl, the enhancement effect increased almost linearly up to a concentration of 100 mm.The appearance of the initial oxygen transient, the burst, was also controlled by the concentration of inorganic cations. Increasing concentrations of MgCl(2) or KCl caused the burst to grow in size in a way which was linearly related to the corresponding increase in enhancement.