The objectives of this study were to define the effect of the milk progesterone (P4) concentration on estrus expression in dairy cows with high milk yield and to identify the effect of milk yield and parity on the milk P4 concentration and cow’s pregnancy after artificial insemination (AI). In this study, 48 clinically healthy cows without reproduction disorders, on day 90-100 after calving were used. At the beginning of estrus and 12 hours after the beginning of estrus, the milk P4 concentration in dairy cows with high milk yield (group 3) was higher than in group 2 (33.66; 22.36%) and in group 1 (51.60; 65.26%) (P 0.05) and the first (28.28%; P < 0.01) lactation ones (28.28%; P < 0.01). The milk P4 concentration at the beginning of estrus and 12 hours after the beginning of estrus in pregnant and non-pregnant cows was 2.58 and 3.32; 4.20 and 5.00 ng/ml, respectively (P < 0.001). As a result, it was concluded that high progesterone concentration affected the expression of estrus and pregnancy results in dairy cows, and the measuring of progesterone concentration in milk can be used as a non-invasive method to provide detailed information about fertility in high milk yield cows
