[en] The objectives of this study were to relate the concentrations of plasma urea nitrogen (UN) to pregnancy rate in dairy cows. The cows were grouped, according to their production level and feed rations according to the NRC recommendations for essential nutrients, Holstein-Friesian dairy cows (n=103), randomly selected, were inseminated between Days 39 and 410 after calving. The cows were separated into two groups, where Group 1, was treated with GnRH i.m. injection on Day 12 after At. Group 2, was not treated and served as a control group. Heparinized blood samples were taken from the tall vein, immediately taken after artificial insemination (Day 0) and subsequently on Days 12, 21, 32 and 55 after Al, respectively. Diagnosis of pregnancy and late embryonic mortality was made by ultrasonography and the measurement of pregnancy proteins (PAG) between Days 32 and 55 after Al. Changes in plasma UN concentrations were studied using multivariate repeated ANOVA procedures. Three cows in each group were insemineted in the lutheal phase therefore their data were not used in the calculation. There were no significant differences between the UN concentrations of the pregnant and non-pregnant cows within the groups. However, the time of sampling had a significant effect on the UN concentrations in both the control and the treated groups. The UN concentrations, in pregnant and non pregnant cows of both groups, were significantly decreasing from Day 0 to Day 32. Concentrations of UN (on Day 0) less than 5,8 mmol/l were associated with increased pregnancy rates (53.6%). The pregnancy rates for cows with UN concentrations within the ranges of 5,7-6,75 mmol/l and 6,76-7,82 mmol/l were 47.8% and 46.7%, respectively. The rate of late embryonic mortality was similar in the two groups (treated group: n=2, control group: n=3).