The occurence of nexuses (gap junctions) in mouse uterine smooth muscle was found to depend on the hormonal state. As revealed by freeze-fracture electron microscopy, nexuses are virtually absent in virginal and pregnant mice. Abundant, large-sized nexuses were observed in the myometrium during parturition. Estrogen application to virginal mice also induced an increase in number and size of nexuses. Our observations indicate that a new formation of nexuses occurs in differentiated cells. New nexuses may be formed by the confluence of nexus subunits preexisting in the cell membrane or by newly biosynthesized components.