The objective of this Study is to examine the effect of the blood urea nitrogen level in cattle on the pregnancy rate. For this, 63 Brown Swiss cows were used as the study material. These cows were grouped into three, each containing 21. and these groups were fed individually over a cycle period with tsoenergic rations containing different crude protein rates. The crude protein rate of the total ration (on dry basis) ted to the study groups was determined to be %14.8, %16.7 and %18.6 for the group 1, 2 and 3 respectively. The cycles of the animals studied were synchronized and the ones having oestrus were inseminated artificially. Uterine pH of the inseminated animals on the oestrus day and the plasma urea nitrogen (PUN) concentrations from the oestrus day to the eleventh day of the cycle period were determined. Pregnancy diagnosis of the animals studied was ultrasonographically performed on the 30-35 th days after insemination. No difference between groups in terms of having oestrus was observed (p>0.05). It was delected that there was significant difference between the PUN concentrations (10.7040.99,17.38±1.20 and 24.26+2.36 mg/dl for the group 1.2 and 3 respectively) of the cows having been fed with the rations containing different crude protein rates (p<0.001). It was seen that the PUN concentrations increased as the amount of the crude protein consumed by the animals increased. It was also detected that there was important difference between the utenne pH values (6.85±0.03. 6.74±0.07 and 6.65+0.08 for the group 1,2 and 3 respectively) on the oestrus day of the cows ted with the rations consisting of different crude prolein rates (p<0.001). The fact that high PUN concentrations decreased the uterine pH on the oestrus day was observed. The pregnancy rates obtained were %80 for the first group. %63.16 for the second group and %30 for the third group. In conclusion, these data suggested that when the crude protein consumption increased the PUN concentrations in cattle went up, also uterine pH decreased on the oestrus day; there was a negative co-relation between the PUN concentrations and the pregnancy rate; and the PUN concentrations over 20 mg/dl in Brown Swiss cows was associated with the decreased pregnancy rate.