The concentration of heavy metals such: Arsenic (As), Lead (Pb), Cadmium (Cd), Mercury (Hg) and Chromium (Cr) was determined in water and sludge of the lagoon La Escondida in Reynosa city on the northeastern part of Mexico by the border with USA. The detection limits by Atomic Absorption Spectroscopy (DL, μg mL-1 ) were 0.05 for As; 0.10 for Pb; 0.10 for Cd; 0.05 for Hg; and 0.30 for Cr in water. The detection limits (DL, μg g-1 ) were 1.0 for As; 10.0 for Pb; 2.0 for Cd; 0.02 for Hg; and 6.0 for Cr in sludge. The objective of this work was to know the levels of these contaminants in this lagoon, since it is surrounded by a highly populated area of the city, besides it is considered as a natural protected resource, in which migratory birds coming from Canada and North America in the winter stop, rest and nest for some time. Nine samples of water and seven samples of sludge from different points, evenly distributed at the lagoon were taken and analyzed; the sampling places were chose according to the currents, influents and effluents of this body of water. The levels of these contaminates were found to be under the detection limits, except for mercury in a single sampling place, which come to be 0.23 μg mL-1 in a sludge sample. This is indicative of water contamination for heavy metals which requires further studies to establish the source and impact of this contamination.