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The objective of this study was to identify the levels of Blood Urea Nitrogen (BUN) and its relation to the reproductive efficiency and serum progesterone concentration in lactating Holstein Friesian dairy cattle. Data were obtained by stratified random sampling from population to get 18 dairy cows, which divided into three groups. The first, second, and third groups each with milk yield of less than 17, between 17-21, and over 21 liters/day, and with calving interval of less than 365, between 365-450 and more than 450 days. Blood was collected from the coccygeal vein at the time of insemination (D0), 7 day (D+7) and 22 days (D+22) after insemination. BUN and progesterone concentration were measured. Data analysis showed no significant difference (P>0.05) on the observed parameters. Data regrouping based on BUN concentration showed that the mean of services per conception (S/C) was lower and conception rate (CR) was higher (P<0.05) in dairy cattle having BUN<18 than those of with BUN>18 mg/dL. The mean of progesterone concentration at D+22 was higher (P<0.05) than those at D0 and D+7 in pregnant dairy cows and with BUN<18 mg/dL, as well as those at D+22 in non-pregnant dairy cows with BUN>18 and BUN<18 mg/dL. This study concluded that high BUN concentration affect the reproductive efficiency of dairy cows by decreasing CR and increasing progesterone serum concentration. [ABSTRACT FROM AUTHOR]