The effects of cobalt chloride administration on the formation of lipid peroxide in the plasma and red blood cells of rabbits, and the alteration of the activity of glutathione peroxidase as well as glutathione content in the red blood cells were studied.Cobalt chloride raised the concentration of the product of lipid peroxidation, malondialdehyde, not only in the red blood cell but also in plasma and caused a slight tendency towards hemolysis. High correlations were observed between the concentration of malondialdehyde and triglyceride and cholesterol in plasma. Neither the activity of glutathione peroxidase nor glutathione content in red cells was affected by cobalt chloride treatment.