We have studied the adaptive response after chronic low dose irradiation (2.5 mGy/h) in wild type Drosophila melanogaster strains (Canton-S and Oregon-R), as well as mutant strains on DNA damage sensing (mei-41), DNA repair (mus209, mus210, mus309, rad54) and free radicals detoxification (sod). The effects of irradiation on the prolongation of the larval stage, pupa lethality, and imago whole body weight have been analyzed. The high dose irradiation (30 Gy, 0.05 Gy/s) induced prolongation of the prepupal period and lethality in all wild type and mutant strains under investigation. The chronic low dose irradiation resulted in shortening of the larval development period (hormetic effect) was observed in wild type (Oregon-R) and mutant (mus209) strains (absorbed dose was 20 cGy) of Drosophila. At the same time these strains demonstrated the hormetic effect after chronic low dose irradiation. The hypersensitivity effect was found in sod and rad54 larvae (20 cGy, prolongation of the prepural period), and rad54 and mei-41 pupa (40 cGy, increase of death rate). The larvae of hypersensitive strains and pupa of all strains under investigation did not have the adaptive response. The chronic irradiation in 6 and 60 cGy with the dose rate of 0.25 and 2.5 mGy/h induced the hormetic effect (imago whole body weight enhancement) Canton-S. The obtained results suggest the important role of free radical detoxification, of DNA damage sensing, and of DNA repair mechanisms in the whole organism radiation induced effects. The appearance of the adaptive response depends on the investigated effect and developmental stage of fly.