The effects of Hg exposure on blood parameters and gill physiology of tilapia (Oreochromis niloticus) were analyzed. Fish maintained in freshwater were exposed for 7 days (d) to sublethal mercury concentrations (0.1 and 1 mg/L). Blood serum osmolality (SO), sodium (Na+), potassium (K+) and chloride (Cl−) ionic concentrations, and hematological parameters were assessed after 1 up to 7 d of exposure. Serum osmolality and ionic concentrations of exposed fish appeared differently affected throughout the experimental period compared to the controls. Osmolality was reduced at the 2 tested concentrations but Na+ and Cl− contents were only altered at 1 mg/L of Hg after 1 d of exposure and values rapidly returned to the control values thereafter. K+ content was also modified and significantly increased at both concentrations after 1 d of exposure but returned to the control values after 3 d of exposure. Red blood cell (RBC), white blood cell (WBC) and hemoglobin (Hb) levels were significantly increased throughout the experiment but returned to control values after 7 d of exposure only for the 0.1 mg/L concentration. The hematocrit (Ht) levels remained unaffected due to Hg exposure. Therefore, tilapias exposed to sublethal concentrations of Hg present a marked osmotic imbalance with ionic and hematological disorders that are rapidly compensated.