Methylation profiles across three CpG islands of the RNASEL gene were determined in blood leukocyte samples of Anglo-Arabian and Hucul horses. Bisulfite sequencing revealed hypomethylated state of the RNASEL promoter coinciding with methylated CpG island placed inside the gene. Several CpG sites were identified for which the methylation state was influenced by DNA polymorphism. Two of them showed monoallelic methylation. One of the CpG sites revealed functional polymorphism. A number of partially methylated CpG sites have been observed in the promoter area of RNASEL, which were used for the comparison of breed- and age-related effects. Clone bisulfite sequencing of blood leukocyte samples collected at different ages from particular individuals of AA and HC breeds and, also, BSPCR sequencing of 50 samples of juvenile and old AA and HC horses revealed increased methylation in particular CpG sites during aging. The age-related heterogeneity of white blood cells was hypothesized as being one of the potential causes of observed variability of methylation profiles in the RNASEL promoter.