When Swiss 3T3 cells are treated with Insulin‐like Growth Factor I, a rapid decrease in the mass of polyphosphoinositol lipids (phosphatidylinositol 4‐phosphate and phosphatidylinositol 4,5‐bisphosphate) occurs within the nuclei, with a concomitant increase in nuclear diacylglycerol and translocation of protein kinase C to the nuclear region. This is in contrast to the effects of the regulatory peptide, bombesin, which causes similar inositol lipid changes in the plasma membrane, has no effect on nuclear inositide levels and causes a translocation of protein kinase C to post‐nuclear membranes. These results suggest the existence of a discrete nuclear polyphosphoinositide signalling system entirely distinct from the well‐known plasma membrane‐located system, which is under regulatory control by cell surface‐located receptors.