Phospholipid composition was studied in tissues of root voles Microtus oeconomus Pall. from natural populations inhabiting territories with increased levels of natural radioactivity. Lipid component of biological membranes of animal tissues was shown to have high sensitivity to action of chronic low-intensity irradiation at small doses. Under effect of various damaging factors on the organism, similar changes of membrane state indexes were noted, these changes being due to functioning of the physical-chemical system of cell metabolism regulation by oxidative reactions in membrane lipids. An important role of intrapopulational processes contributing essentially to formation of biological consequences of the low-intensity irradiation of animals in natural habitat was revealed.