Microelement deficiencies are a current problem in horse breeding, causing infertility and fetal development disorders. The aim of the study was to control the concentration of zinc, copper and selenium in the blood serum of pregnant herd mares. The study included 154 mares in the second half of pregnancy, kept in five regions of south-eastern Poland: Łęczyńsko-Włodawska Plain, Chełmskie Hills, Łukowska Plain, Beskid Niski and Działy Grabowieckie. The concentration of zinc, copper and selenium in the obtained blood serum was determined using atomic absorption spectrophotometry. The serum zinc concentration in all studied mares did not exceed 10.7 µmol/l, while the adopted reference range is 14.9 - 29.2 µmol/l. The lowest concentration of copper and selenium was found in mares from Beskid Niski. In all the studied regions, herd mares had zinc deficiency, and in Beskid Niski additionally copper and selenium deficiency. Despite the identified deficiencies, all the studied mares gave birth on time and registered newborn foals. Since no clinical symptoms of deficiencies of the tested trace elements were observed, it can be assumed that if occurred, they were subclinical.