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Objective To characterize ultrastructurally and biochemically catecholamine release mechanisms of cultured human pheochromocytoma cells in the basal and stimulated states.Methods The cultured pheochromocytoma cells were prepared from human adrenal pheochromocytoma tumors. Biochemical determinations of catecholamine secretion from the cultured cells were carried out in the basal and stimulated states. Transmission electron microscopy was used to observe the modes of catecholamine release from the cells without and with stimulation by depolarization of the cells with the administration of 50 mmol/L KCl.Results Biochemical determinations consistently showed spontaneous secretion of catecholamines from the cultured cells in the basal state without stimulation. Catecholamine release in a calcium-dependent manner could be enhanced in the cells in response to high extracellular potassium concentration. A series of electron microscopic observations of the cultured cells consistently disclosed the classical exocytotic profiles on the cell surface in the basal state. In addition to abundant increase in the number of classical single exocytosis, compound exocytosis was frequently observed in the stimulated cells. Furthermore, other modes of catecholamine release mechanism involving the formation of pseudopodial and/or tubule-like structures, which were different from classical exocytosis, were often present in the intensely stimulation cells. Conclusions Based on the biochemical and electron microscopic findings, we concluded: (1) classical single exocytosis is considered to be a primary mechanism responsible for spontaneous secretion of catecholamines from the cells in the basal state; (2) compound exocytosis is an essential mechanism for extruding large amounts of catecholamines in the stimulated cells; and (3) other modes of catecholamine release mechanism may operate in the cells in response to intense stimulation. These morphological data may be helpful in explanation of biochemical variability and extreme diversity of clinical manifestatons in patients with pheochromocytoma tumor.