Content of nucleic acids as well as activity of DNAases I and II were studied in spleen and thymus cells of linear and hybrid rats with hepatitis. Rhythmic alterations in content of DNA and RNA as well as in the activity of DNAases were detected in spleen and thymus cells within 3 hrs after hepatitis induction. These time-dependent alterations of the patterns studied were dissimilar in linear and hybrid strains of rats. Genetic determination of immune responses appears to be involved in regulation of time-dependent properties of lymphopoiesis, which was responsible, finally, for dissimilar resistance of linear and hybrid rats to stress factors and for development of pathological states.