The cytochemical localization and activity pattern of leucine aminopeptidase (LAP) was studied in the uteri of mature and immature rats in normal and ovariectomized conditions with and without supplementary hormone treatment, as well as in pregnancy and pseudopregnancy. The LAP showed a variable activity pattern and localization specificity in the uterus during the various phases of the reproductive cycle. In the mature animal, moderate LAP activity was localized mainly in the mesometrial region of the endometrium during the diestrous phase and little or no enzyme activity was evident in the uterus during the estrous phase. The activity was greatly enhanced in the mature animal following ovariectomy, the enzyme occurring throughout the uterine stromal tissues. In the immature uterus, ovariectomy resulted in a less significant increase in LAP activity. As in the mature uterus during the estrous phase, LAP activity almost disappeared following supplementary ovarian hormone administration to ovariectomized rats. These results suggest that the enzyme is probably involved in the hydrolytic changes that accompany uterine atrophy. In the pregnant uterus, the distribution pattern of LAP activity was localized at the antimesometrial region in the decidual cells surrounding the implanted blastocyst. Between the implantation sites, the LAP activity was localized mainly in the mesometrial stromal tissue. The gradual increase in activity from the 6th day of pregnancy onwards suggests that the function of the enzyme is probably concerned either with the process of differentiation of the stromal tissue to decidual tissue or with the supply of nutrient to the developing conceptus. The LAP activity in the decidual cells may have resulted from some local factors from the blastocyst itself, as no demonstrable enzyme activity was evident in the pseudopregnant uterus.