The effects of pregnancy or progesterone dominance on the beta-adrenergic responsiveness of the uterus were studied in myometrial membranes from mid- and late-pregnant rats (day 15 and on the 16th h of day 22 of pregnancy respectively) or 24 h after administration of progesterone. Levels of the high (RH)- and low (RL)-affinity states of the beta-adrenergic receptor were determined by competition experiments between 125I-labelled cyanopindolol binding and the selective beta-agonist isoproterenol. The ratio KL/KH (respective dissociation constants) was determined since it also reflects the degree of formation of the high-affinity state of the beta-adrenergic receptor. From day 15 to the 10th h of day 22 of pregnancy, two distinct affinity states were apparent: 80-55% RH (KH = 0.31-0.21 microM) and 45-20% RL (KL = 14-5 microM) with a ratio of KL/KH of 55-34. In the last 6 h before birth, beta-adrenergic receptors underwent uncoupling which was paralleled by decreased responsiveness of myometrial adenylate cyclase to isoproterenol (maximum velocity (Vmax) = 17 +/- 3 vs 44 +/- 3 fmol cyclic AMP/10 min per mg protein on day 15). At this stage of pregnancy, previous exposure to progesterone resulted in a 1.8-fold increase in 125I-labelled cyanopindolol-binding sites (Bmax) and the reappearance of the high-affinity state (67% RH, KH = 0.19 +/- 0.04 (S.E.M.) microM, ratio KL/KH = 81.1 +/- 16.9).(ABSTRACT TRUNCATED AT 250 WORDS)