Exocytotic fusion of secretory vesicles with the plasma membrane occurs in cells in either a constitutive or regulated fashion (Burgess and Kelly, 1987). Constitutive exocytosis is the mechanism by which membrane components and certain soluble proteins are released. In the regulated secretory pathway, secretory products are stored in secretory vesicles until exocytois is triggered by an intracellular signal, and is the pathway for secretion of neurotransmitters and many hormones. The regulated secretory pathways in neurons and secretory cells have a number of aspects in common (Cheek and Burgoyne, 1990), and we and many others have chosen to study the bovine adrenal chromaffin cell as a convenient model system for the investigation of regulated exocytosis (Burgoyne, 1984a; Kink and Knight, 1988; Winkler, 1988). This chapter will concentrate on work on adrenal chromaffin cells (Fig. 1 ) with some reference to exocytosis in other secretory cells, and will cover experimental approaches that have been used in the study of the intracellular regulators and mediators of exocytosis in chromaffin cells. Fig. 1.  General scheme showing the control of exocytosis in bovine adrenal chromaffin cells. The sources of Ca 2+ include entry from outside and release from two separate internal stores. The cytoskeletal barrier at the cell cortex is disassembled in response to Ca 2+ or activation of protein kinase C. The exocytotic machinery is shown as a shaded box. Following exocytosis, vesicle membrane is recovered by endocytosis through coated vesicles. Abbreviations: pIc, phosphoinositidase C; pKc, protein kinase C ACh, acetylcholine; m, muscarinic receptor; n, nicotinic receptor.