Elemental mercury accounts for 10-50% of the dissolved mercury in lakes but plays a vital role in the cycling of this element in lacustrine environments. The view is advanced that a large fraction of the Hg(0) is generated by photochemical reactions in the surface waters. Possible mechanisms for the Hg(0) production are discussed including homogeneous photolysis of dissolved Hg species and photoreduction processes involving organic molecules and inorganic particulates in water.