The concentration of uterine receptors for angiotensin II was investigated during pregnancy in the rat by measuring binding of [l25I]angiotensin II to uterine particles at each day of pregnancy. Uterine angiotensin II receptors changed markedly throughout pregnancy, following a biphasic pattern. During the first half of pregnancy, the concentration of receptors rose by 3-fold from day 2 to day 9 and returned by day 12 to values similar to those of nonpregnant control rats. During the second half of pregnancy, uterine angiotensin II receptors remained below control values until 1 day after delivery. This low binding capacity was not due to occupancy of the angiotensin receptors or to enhanced degradation of the labeled ligand. The major changes in angiotensin II receptor concentration took place in the implantation area, with little or no alterations in the adjacent myometrium. When unilateral salpingectomy was performed on day 2 of pregnancy, the concentration of angiotensin II receptors was still increased on day 9 in the contralateral pregnant horns but did not change in the sterile horns. However, this increase was restored by induction of a deciduoma in the sterile horns. After bilateral salpingectomy, pseudopregnant rats failed to show increase in concentration in uterine angiotensin II receptors between days 5-11, while such rats bearing a deciduoma behaved similarly to normal pregnant rats. Infusion of the converting enzyme inhibitor, SQ 14,225, between days 4 and 9 of pregnancy did not modify the pattern of uterine angiotensin II receptor concentration, indicating that these changes were not related plasma angiotensin II levels. These studies have shown marked changes in the concentration of uterine angiotensin II receptors during pregnancy in rat. These changes are mainly localized in the implantation and appear to be related to the decidualization process. The levels of angiotensin II receptor concentration in the uterus the end of pregnancy exclude a major role for angiotensin the process of parturition in the rat. (Endocrinology106: 1980)