1. Caffeine (20-40 mM) secreted catecholamines from β-escin-permeabilized bovine adrenal chromaffin cells in the presence or absence of 2 mM MgATP. The caffeine-induced catecholamine secretion in the presence of MgATP was to the same extent as that in the absence of MgATP. 2. Ca2+ (0.1-10 μM) induced a significantly greater secretion of catecholamines in the presence of MgATP than in the absence of MgATP. 3. ML-9 (100 μM) and ML-7 (100 μM), myosin light chain kinase inhibitors, and W-7 (100 μM) and trifluoperazine (TFP
30 μM), calmodulin antagonists, inhibited the Ca2+ -induced catecholamine secretion in the presence of MgATP but not in the absence of MgATP. They did not inhibit the caffeine-induced catecholamine secretion in the presence of MgATP. 4. The ATP-independent phase in Ca2+ -dependent exocytosis is thought to be associated with the final step that ultimately leads to fusion, while the ATP-dependent phase is thought to be associated with a vesicle priming reaction. Therefore, these results suggest that the ATP-requiring priming stage is lacking in the process of caffeine-induced exocytosis in bovine adrenal chromaffin cells.