Stability of standard solutions of mercury of concentrations lower than 0.02 mg.l^-1 was examined using various stabilizing agents - HNO₃, HNO₃-HCl-K₂Cr₂O₇ (1 %), HNO₃-KMnO₄ (0.05 %), K₂Cr₂O₇-HNO₃. The Hg concentration in standard solutions was measured immediately after their preparation and after a week. The best results were obtained with HNO₃-HCl-K₂Cr₂O₇ (1 %).