Encouraging results of preclinical pharmacologic analysis prompted the following study on the chronic action of N10-oxide 1-nitro-9-/3-dimethylaminopropylamino/-acridine (C-666) and 1-nitro-9- [/2-dimethylamino/ -1-methylethylamino] -acridine (C-829). Toxic influence of both compounds on liver, kidneys, gonads and intestine epithelium was observed. These histological changes were not, however, followed by any disorders in functions of liver and gonads. Only administration of compound C-666 was followed by the decrease of filtration of renals glomuses. A slight involutional influence of acridine derivatives on lymphopoetic reproductive center of thymus and lymph nodes was observed. At the same time there were no disorders in the value of circular leukocyte system. It was stated that the investigated compounds had no effect on the system of the red cells of the blood; only blood clotting time was prolonged. Compounds C-666 and C-829 did not inhibit the growth of tested animals.