The distribution pattern of laminin in the rat anterior pituitary gland under physiological and hormonally altered conditions was studied immunohistochemically. Intense immunoreactivity of the capillaries and of the basement membranes surrounding parenchymal cells was found. Five to 10% of the parenchymal cells of normal adult rat pituitary gland exhibited also intense positive cytoplasmic staining. These were identified as gonadotrophic cells on the basis of their topographic distribution and typical 700-nm light bodies. By immunoelectron microscopy it was shown that the light bodies contain laminin and tubulin. After treatment with estrogen, which is known to suppress the function of the gonadotrophic cells, virtually no cytoplasmic laminin was found. Ultrastructurally, the number of light bodies in the gonadotrophic cells diminished significantly, from approximately 3 to 8 per cell to 0 to 1 per cell in a given section. In contrast, after castration, the number of laminin positive cells increased to a number above that found in the normal adult male rat, and the number of light bodies increased two to four times. Based on these results, it appears that the presence of cytoplasmic laminin and the number of light bodies reflects the hormonal activity of the gonadotrophic cells of the rat pituitary gland.