Fructose 1,6-bisphosphatase (FBPase)--a key enzyme of gluconeogenesis--for a long time was regarded to be soluble, and freely diffused in the cytoplasm. Our recent investigation revealed however, that in skeletal muscles of mammals, FBPase is located on both sides of the Z-line and, in cardiomyocytes, it is also present inside the cells' nuclei. In the current paper we demonstrate that, in smooth muscle cells, FBPase is located in the cytoplasm and the nucleus, and that the presence of the enzyme in the nucleus is almost completely restricted to the heterochromatin area. In search for additional evidence for the nuclear localization of FBPase and for a possible explanation of its role in the nucleus, we have analyzed the primary structures of muscle FBPases, finding on their molecular surface a number of domains specific for proteins transported into the nucleus.