We examined the extrachromosomal DNA (exDNA, Hirt fraction) in ethidium bromide sensitive and resistant cells of line L929. The exDNA amount is greater in the latter. The amount of exDNA in L929 cells makes 0.19% of the total cellular DNA; the exDNA amounts in cells, resistant to 5 and 50 micrograms/ml ethidium bromide are 0.22 and 0.33%, resp. Using labelling by BudR, it is shown that approximately 16% exDNA in L cells constituted amplified sequences to be excreting to the culture medium. The Zn-independent endogenous nuclease is activated in the resistant cells. The treatment with cycloheximide (50 micrograms/ml) resulted in the increase in the exDNA amount and in the activation of Zn-independent endonuclease. The data obtained suggested that the activation of Zn-independent endonuclease may lead to the increase in the exDNA amount and determine presumably a high rate of cell adaptability to environmental conditions.