AbstractThe rate of volatilization of Hg2+as metallic Hg is accelerated by illumination ofChlorellacells. In the presence of the uncoupler methylamine the rate of volatilization in the light is greatly but transiently increased. DCMU (3‐(3,4‐dichlorophenyl)‐1,1‐dimethyl urea) prevented the light response. In the presence of Hg2+, O2evolution by the cells was not completely inhibited by DCMU. Hg2+appears to prevent DCMU reaching its binding site. Light seems to increase the amount of or leakage from the cells of a metabolite capable of reducing Hg2