The present study was carried out at Sanad Company for Animal Production, Damietta governorate, Egypt, during the period from June to September 2013 (summer). Twenty healthy Holstein dairy cows, free from diseases, of similar age (3 years old) and average body weight (500±25kg) and (80±10 days in milk) were used in the present study. All animals were at the second season of lactation in summer season. Animals were randomly assigned into four experimental groups (each of 5 animals): G1 was intramuscular injected at biweekly intervals with 10 ml saline solution and served as a control group, G2 was intramuscular injected every two weeks with 10 ml viteselen, G3 was intramuscular injected every two weeks with 20 ml viteselen and G4 was supplemented in their daily ration with 3mg, organic Selenium (O-Se)/ kg DMI.  Productive (milk: production, fat, protein, lactose, total solids, solids non-fat and somatic cell count), blood chemical (plasma: Glutathione peroxides, proteins, urea, ALT, AST, glucose, cholesterol and triglycerides) and physiological  parameters (rectal temperature and respiration rate) were measured at the start and the end of 60 days experimental period.
Results indicated that Se treatment either in inorganic or in organic forms with or without vit. E. did not affect significantly all productive, blood chemistry and physiological parameters expect somatic cell count and Glutathione peroxides. It can be concluded that, the main effect of Se treatment is the reduction of somatic cell count due to its effect on immunity; meanwhile, Se had no role in alleviation of heat stress under summer conditions at Northern region (Delta) of Egypt.