In the present study, Caspian brown trout (Salmo trutta caspius) were exposed to environmentally subchronic concentrations of mercury chloride. The effect of this contaminant on blood parameters was investigated. Salmo trutta caspius was exposed to 3.5, 4.8 and 10 μg L-1 of HgCl2 waterborne for 35 days. After 35 days of exposure, Red Blood Cell (RBC) count in three treated groups (1.3±0.02, 1.2±0.02 and 1.1±0.01, 106 mm-3, respectively), Hemoglobin content (9.2±0.3, 8.02±0.3 and 7.1±0.4, g dL-1, respectively), Hematocrit percentage (38.02±2.1, 39.3±2.6 and 34.1±1.8, respectively), Mean Corpuscular Volume (MCV; 257.4±48.7, 172.5±17.6 and 160.9±15.7 μm3, respectively) and Mean corpuscular Hemoglobin (MCH; 72.6±3.2, 61.7±2.5 and 59.6±2.4, pg, respectively) were decreased when compared to the control (RBC; 1.3±0.01, 106 mm-3, Hb; 9.6±0.2 g dL-1 Hematocrit percentage; 38.2±2.2, MCV; 268.7±41.4 μm3 and MCH; 73.5±2.8 pg). The number of white blood cells (WBC; 0.73±0.06, 0.86±0.06 and 0.93±0.09, 104 mm-3) and the Erythrocyte-Sedimentation Rate (ESR; 2.7±1.8, mm h-1) increased in mercuric chloride treated fish. Mean Corpuscular Hemoglobin Concentration (MCHC) values in treated fish (30.7±4.4, 29.6±3.4 and 29.9±5.03%, respectively) were not different compared to control group (31.6±5.6%).