The notochord (Nt) is believed to have a role in the development of axial organs. This study was undertaken to reconstruct in three dimensions (3D) the relationship of the Nt to abnormal development of the foregut (Fg) in the adriamycin-induced rat model of esophageal atresia (EA). Pregnant Sprague-Dawley rats were given 1.75 mg/kg adriamycin intraperitoneally on gestational days 6 9 inclusive; control rats received i.p. saline of equal volume, or no injection. Rats were killed between days 11 and 14 and their embryos harvested, histologically sectioned serially, and stained with hematoxylin and eosin. Digitized photographs were taken of serial transverse sections; these photos were traced and used as the basis for 3D reconstruction. From day 11 the normal Nt is no longer in contact with the respiratory or Fg mesenchyme. In adriamycin-treated embryos the Nt branches abnormally as it enters the Fg mesenchyme. Adherence of the Nt to the mesenchyme of the Fg exerts mechanical traction pulling the upper Fg dorsally. The severity of the Fg abnormalities correlates with the length of the ventral extension of the Nt within the Fg mesenchyme: the embryo develops atresia of the esophagus or trachea when the Nt is grossly abnormal. The Nt undergoes reactive thickening in the absence of Fg structures ventral to it. Thus, structural lesions of the Fg (e.g., atresias) are associated with abnormalities of the Nt. The relationship of the Nt to the Fg mesenchyme determines the severity of the abnormality induced by adriamycin: extensive adherence produces tracheal agenesis and EA.