The ultrastructure of cell and tissue contacts in the developing human pituitary gland were studied in embryos form 3 to 5 weeks of age. At the early stage, the pituitary anlage, Rathke's pouch (RP) of the pharyngeal gut wall, had no direct contact with the diencephalon. At later stages, several close cell-to-cell contact sites were observed between the two tissues, on the upper anterior aspect of RP. However, even at these contact sites a thin layer of extracellular material always remained between the tissues, elsewhere covered by a well-developed basement membrane. RP was, in addition, surrounded by close-coming mesenchymal cells during the time studied. The findings are discussed in reference to different tissue interaction models.