This study was performed to determine the effects of pregnancy on the proportion of T-lymphocytes, null lymphocytes and acid phosphatase (ACP-ase) positive peripheral blood lymphocytes and the distribution of the mentioned cells in the decidua basalis region of endometrium in the 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). The lowest T-lymphocytes percentage was determined at early and middle pregnancy in the peripheral blood (43.83%) and decidua basalis (10.83 number/0.1 mm2), respectively. There was a statistically significant increase in the proportions of the peripheral blood ACP-ase (+) lymphocytes (44.33%) at the mid-gestational period while the lowest ACP-ase positive lymphocyte numbers (5.50 number/0.1 mm2) in the decidua basalis was observed at early pregnancy. The highest null cell rates were found at early gestation either in the peripheral blood (11.50%) or in the decidua basalis (7.83 umber/0.1 mm2). The lowest percentage of peripheral blood lymphocyte (56.00%) was recorded at the early pregnancy. It was concluded that the number and the distribution of the lymphocyte, T-lymphocyte, null lymphocyte, and ACP-ase positive lymphocyte were affected by pregnancy although there were some differences among the gestational periods.