Myometrial biopsies from 80 women, delivered by cesarean section in 26–42 completed weeks of pregnancy, were analyzed using radiolabeled α1-antagonist [3H]-prazosin and α1-antagonist [3H]-rauwolscine. Statistical analysis was performed using Student’s t test (two-tailed), where p < 0.05 was considered significant. The number of α2-adrenergic receptors did not vary (mean value weeks 37–39 =17.2 ± 1.3 fmol/mg protein), while the dissociation constant (Kd) for α2-receptors tended to increase with increasing gestational age, implying reduced affinity near term (p < 0.05, mean values 3.4 ± 0.2 nM). αpRecep-tors, however, increased significantly (p < 0.01) in the pregnancy weeks 37–39 (mean value 29.2 ± 2.9 fmol/mg prot) while Kd was unaltered. Increased number of α1-receptors, along with a decreased affinity of α2-receptors, could, in combination with other factors, play a role in the initiation of labor.