Background. Experimental studies have established that that low-intensity ultrahigh frequency (UHF) electromagnetic radiation (EMR) has a preventive effect under the action of radiation and other pathogenic factors, which manifests itself in the accelerated development of adaptation, in the predominance of anabolic reactions over catabolic ones, and in the strengthening of the body’ structural reserves. At the same time, from the standpoint of studying the mechanisms of primary prevention, it is very important not only to identify the overall adaptation effect, but also to have an idea on the specific contribution of a physical factor used in the development of adaptation. Objective. To identify metabolic and ultrastructural adaption parameters under the action of low-intensity UHF EMR on healthy animals and to determine their stability (safety), by using a model of radiation exposure. Material and methods. The experiments were carried on 42 mature male rats weighing 180—200g. All the rats were divided into 5 groups: 2 experimental groups of 8 animals each for the comparative study of the specific features of adaptation developed under the preliminary action of UHF EMR in healthy animals and under radiation; 2 control groups of 8 animals each, and one intact group of 10 animals. The lumbar region (the area of the projection of the adrenal glands) was exposed to UHF EMP for 2 min daily (a cycle of 10 sessions). The animals were once irradiated at 2 Gy 60Co gamma ray doses, by using an Agat-R apparatus. The investigation objects were the liver, adrenal glands, testes, and thymus. The investigators used biochemical assays (the levels of RNA and DNA, antioxidant activity, specific activity in the thymocyte genome), transmission electron microscopy of adrenocorticocytes in the adrenal bundle zone and Sertoli cells in the testis. Mitochondria (number, average and total area, number of cristae) were morphometrically analyzed. Results. The metabolic and ultrastructural adaption changes induced by UHF EMR in healthy animals were ascertained to be able to withstand the pathogenic effect of radiation and at the same time showed both stability (safety) and certain variability (plasticity) in their reactions. In this case, there were increases in the synthesis of nucleic acids and in the activity of the antioxidant system; there was also mitochondrial structural stability that was more manifested in the adrenocorticocytes in the adrenal bundle zone. At the same time, there was also a decrease in the intensity (magnitude) of a number of adaptive indicators, although by and large they were significantly higher than the level of control (the effect of radiation). Discussion. Thus, the effect of UHF EMR on healthy animals contributed to the development of adaptive rearrangements in the organs studied. The essence of these changes was mainly the enhancement of synthetic reactions, as indicated by a larger amount of RNA and DNA, the activation of cellular and intracellular regeneration with the increased processes of hyperplasia of the mitochondrial cristae and the higher levels of ribosomes and polysomes. In addition, it can be assumed that the use of UHF EMR has laid the physicochemical foundations of its antioxidant action as conformational rearrangements in the membranes, which increased their structural stability. The findings are indicative of the increased level of morphofunctional reserves in the studied cells and tissues due to their increased bioenergetic and plastic potential. Conclusion. This study has provided new data that expand our understanding of the features of the development of adaptive mechanisms and the formation of the body’s structural and functional reserves under the primary prophylactic action of low-intensity UHF EMR, by using the model of radiation exposure.