The inter- and intracellular localization of the plant photoreceptor, phytochrome, was analyzed in tissues of dark-grown rye, Secale cereale L., seedlings by immunogold electron microscopy. Relatively large amounts of immunogold label for phytochrome were found in the epidermal and ground parenchyma of the coleoptile tip and node and cells of the root cap. Lesser amounts were localized in the vascular tissues of the coleoptile tip and node, with little or no label associated with the cells of the apical portion of the first leaf or root proper. In dark-grown tissues staining positively for phytochrome, the label was diffusely distributed throughout the cytoplasm and was not associated specifically with the cell wall or any organelles, membranes, or membrane systems. In dark-grown tissues given a 5-min pulse of red light the immunolabeling of phytochrome occurred both as a diffuse cytoplasmic label and was associated also with electron-dense amorphous regions in the cytoplasm. The electron-dense regions on...