The effect of space flight factors, weightlessness in particular, on the genetic structures of bone marrow cells of rats flown for 18.5 days on Cosmos-1129 was investigated. Chromosome aberrations were measured on postflight days 6 and 25. The frequency of unstable chromosome aberrations was similar in the flight, synchronous and vivarium rats. Karyotyping of metaphase plates revealed chromosome aberrations in the flight and synchronous rats. Exposure to weightlessness did not influence the mutagenic effect in bone marrow cells of the rats.