We used quantitative immunogold electron microscopy to evaluate the subcellular distribution of cytochrome-c in normal rat tissues, employing a wide variety of monoclonal and polyclonal antibodies against mammalian cytochrome-c. Immunogold labeling of tissues embedded in the acrylic resin LR Gold shows highly specific labeling of mitochondria in all tissues examined, including adrenal gland, cerebellum, cerebral cortex, heart, kidney, liver, pituitary, pancreas, skeletal muscle, spleen and thyroid. In pancreatic acinar cells and anterior pituitary, however, there was also strong cytochrome-c reactivity in zymogen granules and growth hormone granules, respectively. In the pancreas, strong immunoreactivity is also detected in condensing vacuoles and in the acinar lumen. Immunocytochemical controls included (i) use of monoclonal antibodies to horse cytochrome-c which recognize an epitope not present in rat cytochrome-c, (ii) preadsorption of antibodies with purified cytochrome-c, and (iii) omission of the primary antibody. The indicated presence of cytochrome-c outside mitochondria in certain tissues under normal physiological conditions raises interesting questions concerning translocation mechanisms and the cellular functions of cytochrome-c.