Salix myrtilloides L. is a relict species in Poland, considered to be particularly endangered, for which it is necessary to carry out active protection through reintroduction and the replenishment of ex­isting populations. The aim of the study was to test the capacity of S. myrtilloides plants obtained in in vitro cultures for effective acclimatization (under both laboratory and field conditions) during the translocation process. In order to trace the response of individuals to changes in environmental conditions on their way from the laboratory to the intended natural habitat, research was carried out on the selected parameters of the functioning of the photosynthetic apparatus (chlorophyll a fluorescence, concentration of photosyn­thetic pigments in plant tissues), the response of plants to environmental stress (accumulation of reactive oxygen species; ROS) and the variability of biometric traits during their growth (rate of daily stem growth). At each stage of the study, verification was carried out on the number of S. myrtilloides plantlets that sur­vived the successive stages of acclimatization and the 12-month growth period in the natural stand. The results of the study reveal that the physiological condition of S. myrtilloides regenerants developed in tissue culture is dependent on changes in environmental conditions during acclimatization, and that the first few days of each stage of this process are crucial and most difficult for the plants. An elevated accumulation of ROS in leaf tissue and lower photosystem II (PS II) efficiency may indicate increased level of stress. Howev­er, these changes may also serve as signals to initiate metabolic, physiological and developmental processes crucial for proper plant acclimatization. Analysis of the results provides important clues for optimising this process by varying the duration of the various stages.