The rate of volatilization of Hg2+ as metallic Hg is accelerated by illumination of Chlorella cells. 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+, O2 evolution 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+ to Hg°.