Bovine chromaffin cells cultured for 5 days in the presence of depolarizing concentrations of K+ ions show a decreased number of secretory (chromaffin) granules per cell. These cells were still capable of exocytosis. Their contents in catecholamine and chromogranin A, components of the granule matrix, and cytochrome b561, a major protein of the granule membrane, were decreased to 35, 30, and 50% of control cells, respectively. However, in the same cells, the number of [3H]dihydrotetrabenazine binding sites, a specific ligand of the vesicular monoamine transporter, was increased to 180% of controls. In situ uptake of noradrenaline in permeabilized cells indicated that [3H]dihydrotetrabenazine binding sites were associated with a functional vesicular monoamine transporter. When analyzed by isopycnic centrifugation, these sites cosedimented with catecholamine, chromogranin A, and cytochrome b561, in a peak with a density lighter than that from controls. The composition of this peak suggests that it contains incompletely matured secretory granules, with a 3-5-fold increase in the vesicular monoamine transporter content of this membrane. This increase might indicate that an adaptative process occurs which allows a faster filling of the granules in continuously secreting cells.