This thesis explores the complex influence of environmental factors on the: 1)annual, seasonal, and circadian spatial distribution, 2) annual home range size (HR), and 3) body mass of red deer. Analyses of red deer habitat selection and HR are based on multiyear VHF telemetry data gathered from 50 hinds and stags (11.000 fixes) and a one-year GPS telemetry of 4 hinds (16.000fixes) from 4 different areas within Slovenia. Furthermore, analyses ofred deer body mass are based upon geo-referenced (1 km accuracy) datasets of3.920 deer harvested from the entire land area of Slovenia. All spatially explicit habitat features (e.g. topography, vegetation type, forest structure,roads, supplemental feeding places - SFP) and other environmental factors (e.g. red deer harvest density, temperature, participation) were incorporated into a GIS. The main results can be summarized as follows: 1) Reddeer aggregate around SFP throughout the year, especially during winter. Supplemental feeding also diminishes HR, but there was no effect on red deer body mass caused by this measure. Red deer density was strongly increased within a 500-meter radius of the SFP and still noticeably increased within a 1500-meter radius - a fact that needs to be considered because of heavy forestdamage in these areas. 2) Since red deer primarily use the vicinity of the forest edge, the species can be viewed as ecotonal. Non-forest areas (meadows, agricultural areas) provide crucial feeding habitats, and their availability influences the carrying capacity of habitat As a result, the increase of non-forest areas and forest edge density decreases the red deer HRand increases its body mass. 3) Red deer avoids the proximity of settlementsand roads (250 m min.) and stays concealed in their vicinity (increased use of security cover, exclusively nocturnal use of nearby areas). Due to their barrier effect, roads functionally cut red deer potential habitatinto fragments, which also affects the HR size (the latter positively depends on fragment size). 4) Deer prefer forests with richer herb and shrub layers (young forest, selective thinning forest, forest in rejuvenation phase etc.) since they offer more food and security cover. Mature stands and especially pole stands are less preferred. As the percentage of less preferredforest (as well as that of conifers) increases, red deer body mass decreases. 5) As elevation increases (resulting in a shorter vegetation periodand prolonged winter), red deer body mass decreases and its HR size increases. 6) Body mass and HR size of red deer is also influenced by its population density. 7) The red deer can thus compensate for the spatial variation of habitat quality through its specific spatial distribution and theuse of smaller/larger HR, despite that it does not reach an ideal free distribution, therefore habitat quality also influences its body mass.