Blood plasma profiles of equine sex steroids are useful in diagnose and for different scientific purposes. The aim of this study was to monitor oestradiol, testosterone and androstenedione blood plasma profiles in foals of age between 6 and 18 months, in order to establish the reference levels for male and female of young Konik Polski horses. The effects of birth place (stable vs. forest), age and season. Fifty two Konik Polski horses, born in the forest reserve (n=26) and in the stable (n=27)were studied.Level of oestradiol differed between sexes at the age of 12 months, whilst androgens were higher in males of every age group. At all ages the level of oestradiol, testosterone and androstenedione did not differ between stable and forest born colts and fillies, except for fillies at 18 months of age. The highest level of oestradiol in both colts and fillies was found at the age of 15 months, i.e. at the late summer. In colts, testosterone and androstenedione concentrations were at their nadir at 6 and 9 months of age, then they rose abruptly at month 12 of age and remained significantly elevated until month 18. In fillies, the highest concentrations of these androgens were found in August-September,when the females were 15 months old. The profiles and the role of sex steroid hormones in social and reproductive behaviours are discussed.