Summry—The subcellular partitioning of the phosphoinositidase C (PIC) isoforms involved in signal transduction, with the selective localization of the PIC β1isoform in the nucleus, represents a crucial aspect of the complex mechanism of cell response to agonists. In order to further elucidate this phenomenon, we utilized human osteosarcoma Saos‐2 cells, transfected with the cDNA for rat PIC β1. In the cells overexpressing this isoform, immunocytochemical analyses at the electron microscope level reveal an increased synthesis at the cytoplasm and a significant accumulation within the nucleus of the protein. Interestingly, the sites of intranuclear localization are, as in wild type cells, the interchromatin domains. These results indicate that the transfected cells maintain the capability of accumulating the enzyme within the nucleus and can be considered a model for functional studies on the nuclear signal transduction also in response to specific ago