The human placental bed myometrium, studied in 42 intact hysterectomy specimens ranging from 8 to 18 weeks' gestation, is characterized by the presence of large numbers of non-villous invasive cytotrophoblastic cells. Quantitative morphometric analysis reveals a tendency for maximal invasive activity to occur at the centre and, subsequently, to extend centrifugally to produce an annular pattern. Morphological observations suggest that the intimate mixture of cytotrophoblast with myometrial tissue must affect the mechanical properties of the myometrium. Local hormone production by trophoblast may induce or enhance these and other changes in uterine tissues that are essential for the establishment of human placentation. Cytotrophoblastic invasion into the myometrium appears to be restricted to the earlier stages of gestation and morphological evidence indicates that, subsequently, clumps of cytotrophoblast fuse to form multinuclear syncytiotrophoblast, the characteristic placental bed giant cells.