The objective of the study was to determine the content of selenium in selected tissues of slaughter horses and to define its variability depending on the age and sex of the animals. The research material consisted of three muscles and internal organs (kidneys, lungs, and liver) from 117 carcasses of slaughter horses classified into the second quality class. The experimental material selected comprised animals of two sexes and of two age groups: a group of young horses (6 to 18 month old) and a group of old horses (aged between 10 and 13 years). The samples taken were initially prepared and, next, mineralized. The content of selenium in the mineralisates produced was determined using an atomic absorption spectrometer with inductively coupled electro thermal and deuterium background correction (Zeeman, Varian Spectra 220Z). Significant differences in the contents of selenium were proved to exist among all the tissues tested. The highest content of selenium was found in the kidneys (0.487 ppm), a significantly lower content of that element was determined in the liver (0.177 ppm), and even lower in the lungs (0.062 ppm). The muscle tissue contained the lowest amount of selenium (0.039 ppm). When considering the age of the horses, a statistically significantly (p ≤ 0.01) higher amount of selenium was found in the kidneys and muscles of the young animals. The sex had a minor effect on the content of selenium both in the internal organs and in the muscles. The statistically significant (p ≤ 0.01) differences were found only in the kidneys. Of the three examined muscles, the highest levels of selenium were determined in the supraspinatus muscle, a bit lower but statistically significant (p ≤ 0.01) amounts in the longissimus and semimembranosus muscles. Those differences were found only in the muscles of the young animals. Furthermore, in that group, significant (p ≤ 0.01) differences were found in the content of selenium depending on the type of muscle and sex of the animals. The muscles of the animals aged more than 10 years contained the lowest level of selenium. In that group of animals, no significant (p ≤ 0.01) differences were found among the muscles analyzed; no significant impact of the sex on the content of selenium was found either.