Single bovine adrenal medullary cells have been obtained by retrograde perfusion of adrenal medullae with a solution of 0.05% coUagenase in Ca §247 free Krebs Henseleit buffer. Chromaffin cells were obtained in high yield (5 x 106 cells/g medulla), and more than 95% of these were viable as shown by exclusion of trypan blue. The isolated cells were capable of respiring at a linear rate for a minimum of 120 min.Ultrastructural examination revealed that the cells were morphologically intact, and two distinct types of adrenal medullary cells were identified, on the basis of the morphology of their electron-dense vesicles, as (a) adrenalinecontaining and (b) noradrenaline-containing cells.Biochemical analysis showed that the cells contained catecholamines and dopamine-/3-hydroxylase (DBH). The cells released catecholamines and DBH in response to acetylcholine (ACh), and this release was accompanied by changes in the vesicular and surface membranes observed at the ultrastructural level. The time-course of ACh-stimulated catecholamine and DBH release, and the dependence of this release on the concentration of ACh and extracellular Ca + § have been investigated. The isolated cells were pharmacologically sensitive to the action of the cholinergic blocking agents, atropine and hexamethonium.