To study the role of epithelial-mesenchymal interactions in myometrial development, uteri from neonatal Balb/c mice 1 to 60 days postpartum were utilized. Intact (untrypsinized) uteri, trypsinized but unseparated uteri, homotypic uterine tissue recombinants (separated-recombined), or uterine mesenchyme alone were grafted beneath the renal capsule of syngeneic female hosts and grown for 1 mo. Uterine mesenchyme from 1-day mice grafted alone produced small amounts of smooth muscle, most of which was associated with vasculature, whereas uterine mesenchyme from older donors possessing a rudimentary myometrium at the time of grafting formed intermediate amounts of myometrium (actin-positive smooth muscle bundles). In contrast, all specimens containing epithelium (intact, trypsinized, and separated-recombined) developed large amounts of myometrium. Uterine epithelia from neonatal through adult stages were equally effective in permissively inducing myometrial development in 1-day uterine mesenchyme. From these data, it is apparent that uterine epithelium plays an important promotional role in the differentiation and possibly the spatial organization of the myometrium.