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Metallic mercury has been assumed by several authors as not very reactive and, as a consequence, with little or no toxicity. The toxicity of this element is usually ascribed to alkyl-mercury ions considered to be formed by some microorganisms. In this work, we describe experiments that clearly show that metallic mercury can be easily oxidized by molecular oxygen in aqueous solution in the presence of species such as chloride, which complex Hg(II). The experiments were carried out using metallic mercury in NaCl aqueous solution under 'open air' (temperature and agitation rate maintained constant) and under more controlled conditions (CO2 rate bubbling, i.e. pH = 4.2; air rate bubbling, i.e., O2 constant concentration, temperature, agitation rate). The reactions were monitored spectrophotometricaly at 230 nm (HgCl2-(4)). Significative values of the concentration of Hg(II) in the form of HgCl2-(4) were soon attained in those solutions. For example, in 'open air' conditions, at 25 degrees C and [NaCl] = 30 g/l (0.51 mol/dm3), the maximal concentration of 13 ppm (6.44 x 10(-5) mol/dm3) of Hg(II) in the form of HgCl2-(4) was reached in 120 min; for [NaCl] = 5 g/l at 25 degrees C, (0.085 mol/dm3) the maximal concentration of 0.3 ppm (1.53 x 10(-6) mol/dm3) of Hg(II) in the form of HgCl2-(4) was reached in 10 min. The rate constants, kobs, of the oxidation of the metallic mercury under the studied conditions are pseudo zero-order at 25 degrees C, and under more controlled conditions have ranged from 1.0 x 10(-7) mol/min ([NaCl] = 5 g/l identical to 0.085 mol/dm3) to 20.0 x 10(-7) mol/min ([NaCl] = 300 g/l identical to 5.12 mol/dm3). The rate constant increases with temperature, up to 25 degrees C, from where kobs remains constant up to 40 degrees C. From the analysis of the experimental results it was possible to propose a mechanism of oxidation of metallic mercury by O2 in aqueous solution containing NaCl. This oxidation is proposed as a possible route for the introduction of mercury into biological systems.