Stimulation of aldosterone secretion by increases in extracellular potassium concentration is associated with increases in the volume of the adrenal glomerulosa cell. Because increases in cell volume have been associated with increases in aldosterone secretion, the effect of preventing the potassium-induced increase in cell volume by removal of chloride from the medium on the response of dispersed bovine glomerulosa cells grown in primary culture was determined. Totally replacing the chloride ion with the methylsulfate ion prevented the increase in cell volume and significantly suppressed the increase in aldosterone secretion normally associated with increasing [K] to 10 mM. In the absence of Cl-, the increase in cytosolic calcium concentration ([Ca2+]c) normally induced by increasing the [K] to 10 mM was also significantly suppressed. The replacement of 10 mM methylsulfate by Cl- restored the potassium-induced increase in both cell volume and aldosterone secretion to values not different from those found in the presence of 108 mM Cl-. The potassium-induced increase in cell volume was dependent also on the presence of extracellular calcium. Thus a component of the glomerulosa cell response to an increase in [K] may be caused by a chloride-dependent increase in cell volume that is triggered by the initial depolarization-induced increase in [Ca2+]c. The increase in cell volume enhances the increase in [Ca2+]c and amplifies the increase in aldosterone secretion.