To gain insight into the mechanisms of synthesis, storage, and release of basic fibroblast growth factor (bFGF), we studied the immunohistochemical localization of bFGF in bovine coronary artery, coronary sinus, and adrenal capillary endothelial cells grown in culture. Light and electron microscopic immunohistochemical studies were performed using the ABC immunoperoxidase method on p-formaldehyde-fixed cells. Five different anti-bFGF antibodies gave similar results in all cell types. In subconfluent cells, immunoreactivity was noted in the nuclear chromatin, nucleoli, cytosol, cytoplasmic vesicles (some of which appeared to fuse with the plasma membrane), and extracellular matrix. No reaction was found in endoplasmic reticulum or Golgi zones. Confluent cells demonstrated less immunoreactivity in the nuclei and cytosol but more in the extracellular matrix. Some cells of senescent morphology showed only cytoplasmic staining; however, no cells were found with only nuclear staining. Biochemical studies showed that three forms of bFGF (18, 24, and 26 kDa) were present in endothelial cells and varied with different culture conditions. Protection analysis indicated that bFGF mRNA is less abundant in postconfluent cells than in subconfluent cells. These data suggest that subconfluent cells synthesize bFGF and transport it into the nucleus and exocytotic vesicles, while confluent cells synthesize little bFGF but store it in extracellular matrix, cytoplasmic vesicles, and nuclei.