The subcellular distribution of catalytic (C) and regulatory (RI and RII) subunits of cAMP-dependent protein kinases has been studied by electron microscopy immunocytochemistry. The C-subunit was localized in the inner membrane-matrix space of mitochondria, at the cytoplasmic face of the rough endoplasmic reticulum (rER), in the nucleolus and in peripheral heterochromatin regions. A C-specific immunoreactivity was also found in specific domains of the basal and basolateral plasma membranes of acinar and duct cells, in centrosomes and on keratin filaments which anchor in desmosomes. The RI- and RII-subunits showed a basically similar subcellular distribution. A remarkably high RII-immunoreactivity, in the absence of C-immunoreactivity, was demonstrated in the secretory granules. These results demonstrate the presence of cAMP-dependent protein kinases or their subunits in many subcellular organelles. They also indicate a role for cAMP-dependent protein kinases in the regulation of a number of basal cellular functions as well as their importance in functional and structural cell-cell interactions.