This study was performed to determine the effects of pregnancy on the activities of alpha naphthyl acetate esterase (ANAE) and acid phosphatase (ACP-ase) of the peripheral blood lymphocytes in pregnant Merino sheep. Peripheral blood lymphocyte percentages were also estimated. Periferal blood samples were taken from animals in six different gestational stages as non-pregnant control, in the first, the second, the third, the fourth and the fifth month of pregnancy. Each group was contained 20 animals. The lowest ANAE (+) lymphocytes percentage (63.5%) was determined in the first month of pregnancy whereas the highest null lymphocytes proportion (12.75%) was detected in the same gestational period. There were statistically decreases in the proportions of peripheral blood lymphocyte (42.9%) and the ACP-ase (+) lymphocytes (43.35%) in the last gestational stages. Although the posibble hormonal changes may cause the fluctuation of peripheral blood lymphocyte proportions in all gestational periods, the most distinctive changes were observed at the begining and at the end of the pregnancy. It was concluded that the data was obtained from this study was useful for understanding of the possible mechanisms of maternal tolerance.