Two organic compounds of selenium, 4-o-totyl-selenosemicarbazide p-chlorobenzoic acid (chain compound) produced at the Chemistry Department of the University Medical School in Lublin, and one inorganic compound of sodium IV selenite (Na2SeO3) were used. The preparations were used per os in doses of 1 mg/kg body weight and 0.5 mg/kg body weight. The studies were conducted on female Swiss mice, covering seven groups of animals, i.e. 6 experimental and 1 control. Histopathologic changes were observed in liver, kidney, lung and heart. Ultrastructural changes were observed in liver and kidney. Our studies indicate a dose-dependent effect of selenium on histopathologic and ultrastructural changes. It is possible therefore, that the extent of excess of selenium exerts a greater influence on a cell than the form of supplemented selenium