EN
The level of progesterone and 17 β-estradiol was determined in plasma of cows at 0, 3 or 4, 7 and 21 day of oestrus. It was found that 6,3% of cows in a phase of active yellow body was declared to inseminate. No differences in the level of progesterone at day 0, 3, 4 and 7 between groups of pregnant and non-pregnant cows were found. At days 7 of oestrus an extrectioin of the yellow body was not fully developed because the level of progesterone increased twice in pregnant cows between 7 and 21 day. Out of 7 cows revealing at day 21 a high level of progesterone and sterile in 3 deat of embryos can be supposed. The results of the determination of 17 β-estradiol varied both for individual days oestrus and for the presence of various ovarial structures. The determinations of the level of this hormone appeared to be unuseful in individual evaluation of causes of sterility. In the contrary, for this reason is valuable determination of the level of plasma progesterone.