EN
The paper presents the results of the research on the karyotype structure and amount of nuclear DNA in reciprocal hybrids between the tetraploid alpine cats tail grass and the tetraploid timothy grass. The hybrid forms were grown from Fl grains. For the karyotype analysis staining in acetic orceine and C-banding were used. Nuclear DNA measurements were taken in a flow cytometer. In the root apical meristem of the two forms the authors revealed the occurrence of an unstable chromosome number — apart from the typical tetraploid number 2n = 28, different aneuploid numbers were recorded. The band karyotype shows the occurrence of the same telomeric type ofheterochromatin distribution, characteristic of the genomes of one of the parental forms — P. pratense [4x], no matter if it was a maternal or paternal form in the crossing.