Pregnancy in sheep is associated with changes in numbers of specific T-lymphocyte populations in the uterine endometrium. These changes probably contribute to evasion by the conceptus of maternal immunological rejection and indicate a possible role for T cells in placental growth, parturition and post-parturient uterine defence against infection. The purpose of the present experiment was to evaluate the relative importance of systemic signals (i.e. those present throughout the uterus or from the circulation, including conceptus hormones secreted into the maternal blood) versus locally acting conceptus signals for regulating changes in numbers of endometrial lymphocytes during pregnancy. The approach taken was to surgically confine pregnancy to one uterine horn and compare differences in lymphocyte numbers between the two uterine horns as well as between both horns of pregnant ewes with those of ovariectomized ewes. As compared with ovariectomized ewes, there was a decline in numbers of CD45R+ lymphocytes within glandular epithelium and an increase in γδ T-cell number within the luminal epithelium. These changes occurred in both the pregnant and non-pregnant uterine horns of unilaterally pregnant ewes. Moreover, there were no significant differences in lymphocyte numbers between the two uterine horns of unilaterally pregnant ewes. Expression of CD25 was absent in tissues from both uterine horns. In conclusion, changes in numbers of endometrial lymphocytes during pregnancy, rather than due to locally acting signals of conceptus origin, are the result of hormonal signals of maternal or conceptus origin that either act directly on endometrial lymphocytes or stimulate the uterine endometrium to induce synthesis of regulatory molecules that affect lymphocyte dynamics.