The activity of enzymes involved in DNA disintegration (acid and alkaline DNA-ases) was measured in the spleen of rats flown onboard the Cosmos-605 biosatellite for 22 days. The activity of alkaline DNA-ase remained unaltered whereas that of acid DNA-ase increased more than twice. The activity of acid DNA-ase returned to the normal 26 days postflight. The study of characteristic viscosity (eta) of native and denatured DNA showed that enzyme activation was not followed by degradation changes in the DNA molecule.