The aim of the investigation was to determine influence of dietary supplementation with selenium on blood metabolic profile and thyroid hormones activities in fattening lambs. The study included 36 Merinolandschaf lambs during fattening period (50 days) divided into three groups (control, Exp-I and Exp-II). The control group diet was not supplemented with Se, whereas the diet of Exp-I was supplemented with 0.3 mg/kg inorganic selenium (sodium selenite) and Exp-II with 0.3 mg/kg organic selenium (Sel-Plex). Concentration of minerals (Ca, P-inorganic, K, Na, Fe and Cl), biochemical indicators (urea, glucose, cholesterol, HDL-cholesterol, LDL-cholesterol, triglycerides, creatinine, total protein, albumin), enzyme activity (ALT, AST, GGT, LDH and GSH-Px) and thyroid hormone activities (T3, T4) were determined. Significantly higher (P less than 0.01) selenium content and GSH-Px activity as well as the lower content of Ca was determined in the blood of Exp-II and Exp-I groups compared to control group (93.89 and 67.69: 34.11 mug L-1; 607.98 and 556.92: 247.52 mukat L-1; 2.38 and 2.39: 2.76 mmol L-1). Significantly higher (P less than 0.01) selenium content in the in blood of lambs from Exp-II compared to Exp-I group was determined. Significantly higher (P less than 0.05) concentrations of HDL-cholesterol and T3 hormone activity were found in Exp-II group compared to the Exp-I and control groups. The results showed lower blood selenium content in control group and indicate the justification of adding selenium, especially organic selenium in lamb's diets, raising the concentration of Se, GSH-Px enzyme, HDL-cholesterol and T3 hormones improving the health status, especially in areas deficient in selenium.