Lymphomas belong among the most frequently diagnosed tumours of the haematopoietic system in dogs. The clinical manifestations and genetic and molecular basis of canine lymphoma resembles those of human non-Hodgkin lymphoma and therefore it can serve as a suitable model for the study of this disease. Neoplastic diseases are the consequence of a number of genetic and epigenetic changes in somatic cells. One of such changes are gene mutations that can subsequently cause changes in the activity of proto-oncogenes and tumour suppressor genes. The aim of our study was to detect potential mutations in selected exons of proto-oncogenes in DNA isolated from samples of lymphoma obtained from two donors - a Bernese Mountain Dog and a female mongrel. On the basis of literary data descriptions of human and canine haematopoietic neoplastic diseases, our investigations of potential changes in DNA focused on proto- oncogenes C-KIT - exons 8, 17; NRAS - exons 1, 2;FLT3 - exons 14, 15 and 20. The investigated samples were amplified by polymerase chain reaction (PCR) and subjected to sequencing. The DNA sequences were compared with reference sequences in the database Ensembl. The comparison of sequences of the C-KIT gene revealed an A/G transition at the 35th nucleotide of exon 8 in the mongrel. It involved a synonymous exchange of the nucleotide in the codon that did not cause a change in the amino acid. In the same sample we recorded several point mutations in the intron regions surrounding the exons 14 and 20 of the FLT3 gene. Changes in the intron regions can affect the expression of genes and thus can play an important role in the origin and development of tumours. No genetic mutations were detected in any gene regions of the Bernese Mountain Dog. In the case of the NRAS gene, no changes were observed in any sample collected from the donors.