Hormones were localized immunoenzymocytochemically at the ultrastructural level directly on ultrathin sections of anterior pituitary glands of rats which had been fixed and embedded in either methacrylate or Epon. GH, LTH, ACTH and LH are best localized on methacrylate embedded glands and GH and LTH on Epon embedded glands. GH and LTH were found in secretion granules, and depending on the activity of the cells, the hormones could be found in the Golgi apparatus and in the cisternae of endoplasmic reticulum. The grids on which hormones have been localized may also be processed for electron radioautography, an approach particularly useful to study simultaneously substrate uptake as well as product synthesis.