The aim of this thesis was to study the development of Brown cattle in Slovenia on the basis of historical written sources and to study the structure of the current Brown cattle population on the basis of mtDNA sequences. The Brown cattle population is genetically heterogeneous population. Based on the written historical sources, the current population is based on the ancient local cattle, which were first improved with Grey cattle, later with Original Brown cattle, and at present time with Brown Swiss cattle. Depending on the maternal inheritance mtDNA it was assumed that some haplotypes of mtDNA from the breeds used for improving the Brown cattle population in Slovenia could be found within Brown cattle population. The study included 59 samples of the biological material of cattle. We were able to obtain PCR products from only 52 samples, of which only 23 samples produced readable sequences. All 19 samples of Brown cattle from Slovenia were classified in haplogroups T, T1, T2 and majority of them in T3, where the samples of Brown Swiss cattle from Canada are placed as well. In addition to these two breeds, Modicana, Piemontese and Grigio Alpina are also classified in the haplogroup T3.