The capacity of testosterone administered in vivo in the form of testosterone propionate (TP), to modify chromosome integrity and a search for certain conditions under which the hormone manifested cytogenetic activity, were determined. TP was administered at a single dose or for 12 days at various doses to random bred albino male rats aged 3-4.5 mos. Single administration of the hormone to animals aged 4-4.5 mos. did not result in an increase in the number of hepatocytes with chromosome aberrations even at a dose exceeding 6-7-fold daily production. However after daily administration for 12 days at a dose exceeding only twice the daily one, TP increased 1.5-fold the number of cells with damaged chromosomes. In animals aged 3-3.5 mos. the same scheme of administration of TP turned out ineffective. The results obtained are under discussion.