This study was carried out to determine the distribution of uterine natural killer (uNK) cells having periodic-acid Schiff (PAS) positive granules in the decidua basalis in pregnant mice. For this purpose, mice at 12-14 weeks of age were divided into four groups as non-pregnant control, and at the middle of the first, second, and the third week of the pregnancy, corresponding to early, middle, and late (3rd, 10th, and 17th days of pregnancy) gestational stages respectively (n= 6 for each group). After placentation, it was identified four different regions as metrial region (MLAp; mesometrial lymphoid aggregate of pregnancy), decidua basalis, junctional zone and labyrinth zone in mice. It was observed trophoblast giant cells between decidua basalis and junctional zone. The number of PAS-positive uNK cells was found as 5 in 10.000 μm2 in endometrium of control animals while it was determined as 26.5 number/10.000 μm2 in decidua basalis at early gestational period. The highest PAS-positive uNK cell numbers (56 number/1.0000 μm2) were detected at the mid-gestational period in decidua basalis. uNK cells were also observed in the lumen of the maternal blood vessels. In mesometrial region, the dilatation of maternal blood vessels and thinning of the media layer was distinct. It was concluded that the number and the distribution of the uNK cells having periodic-acid Schiff (PAS) positive granules was affected by pregnancy.