Oxytocin receptors (OXT-R) and prostaglandin F_2α receptors (PGF_2α-R) in human myometrium, amnion and decidua during pregnancy and at parturition were examined in an effort to clarify their role in the initiation and maintenance of uterine contractions. The number of binding sites for OXT in myometria showed an increase as gestation advance (Ist trimester v.s. at term ; 205+90 v.s. 671 ± 98 fmol/mg protein, N=5, p<0.01), and a rapid decrease following the onset of labor (254 ±60 fmol/mg protein, N=5, p<0.02). On the other hand the number of PGF_2α-R, remained unchanged throughout pregnancy and in labor. This myometrial PGF_2α binding capacity was approximately 1/20 to 1/30 that of the OXT binding, while binding affinity was almost equal. The OXT-R both in amnion and decidua, which was 1/6 to 1/7 that in myometrium, showed no significant changes throughout pregnancy or after the onset of labor. Binding affinity for each tissue was almost the same and appeared to increase towards term but no statistical significance was detected. Present data confirmed the presence of OXT as well as PGF_2α receptors in the three functionally distinct entities of pregnant human uterus ; myometrium, amnion, and decidua. Among the components, the OXT binding increased only in the myometrium during pregnancy, suggesting this tissue specifically responds to OXT. In contrast, there was a constant binding in myometria for PGF_2α. Further, in in vitro incubation of the tissue with PGF_2α, the predominant change observed was an increase in OXT-R binding affinity. This change was observed in all these specimens at term before labor. Since human uterine contractions can be induced effectively with PGF_2α throughout pregnancy while induction by OXT is only feasible near term, our observations seem to explain the differences in efficacy of the two uterotonic compounds at different stages of pregnancy.