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Different types of integrin alpha subunit were investigated immunohistochemically to determine the relationship between the cell adhesion molecule and the local morphology of the rat anterior pituitary using anterior pituitary tissues and primary cultured cells. Alpha 3 subunit was shown to be the main integrin subunit in the anterior pituitary. In vitro, immunoreactions of α3 subunit appeared mainly surrounding cell clusters. The distribution roughly overlapped with immunoreaction of laminin, while it was also observed where expression of laminin was negative. When dispersed cells were primary cultured for 2, 3 or 5 days, immunoreaction of α3 subunit was clearly detected in the vicinity of cell membrane throughout the period; while that of α5 subunit was not observed on day 2, it was observed as dots in cytoplasm on day 3 and more distinctly as lines in the vicinity of the cell membrane on day 5. Alpha 5 subunit was not detectable in vivo. These results indicate the possibility that integrin α3 subunit not only binds the cells to the basement membrane but also to other extracellular matrix or directly to adjacent cells in the anterior pituitary. In addition, integrin α5 subunit may supplement the function of α3 subunit under certain conditions.