Intestinal absorption of copper is inhibited by zinc. The aim of this study was to define the maximal value of Zn:Cu ratio in feed ration sufficient to increase the supply of Cu in mares with diagnosed Cu deficiency. The research was conducted in the stud, in which 46 percent of broodmares remained barren because in the previous reproductive season they had shown various reproduction malfunctions: the lack of estrus and ovulation, silent estrus, the lack of pregnancy after insemination as well as fetal resorptions on the 21st–35th day of pregnancy. The levels of Cu, Mn, Fe and Zn were determined in blood plasma from 10 barren mares and in fodder samples by atomic spectrophotometry absorption (ASA). During the study two common mineral supplements were given in doses recommended by the manufacturers, which decreased the Zn:Cu ratio in fodder from 9.0 to 8.3 or 6.0. When Zn:Cu fodder ratio was 8.3, no effects of eight weeks of supplementation were shown. After decreasing this ratio to 6.0, plasma Cu level in mares increased from 5.5 to 12.1 mol/l and the mating period has gone without disturbances since then.