Membrane homeostasis is maintained by exocytosis and endocytosis. The molecular mechanisms regulating the interplay between these two processes are not clear. Inositol hexakisphosphate (InsP₆) is under metabolic control and serves as a signal in the pancreatic β cell stimulus-secretion coupling by increasing Ca²⁺-channel activity and insulin exocytosis. We now show that InsP₆ also promotes dynamin I-mediated endocytosis in the pancreatic β cell. This effect of InsP₆ depends on calcineurin-induced dephosphorylation and is accounted for by both activation of protein kinase C and inhibition of the phosphoinositide phosphatase synaptojanin and thereby formation of phosphatidylinositol 4,5-bisphosphate. In regulating both exocytosis and endocytosis, InsP₆ thus may have an essential integral role in membrane trafficking.