1. Intracellular recordings were made from chromaffin cells isolated from adrenal medullae of gerbils to examine the effects, on membrane potential, of changes in the ionic environment that are known, from other experiments, to influence the rate of catecholamine secretion.2. Depolarization in response to acetylcholine fell linearly with the logarithm of the extracellular sodium concentration over the range 154-3 mM and reached a value, in sodium-free medium, of about 30% of the control value.3. The depolarizing effect of acetylcholine in sodium-free media increased linearly with the logarithm of the extracellular calcium concentration over the range 1-117 mM. It is concluded that depolarization in response to acetylcholine involves inward movement of both sodium and calcium ions.4. Depolarization was also observed in response to the secretagogues, excess potassium and barium, both in sodium-rich and sodium-free media. The effect of barium was antagonized by calcium, and it is suggested that these two cations interact at the level of the plasma membrane.5. Depolarization does not appear to be tightly coupled to secretion, for acetylcholine or excess potassium still depolarized the chromaffin cells when the environment was calcium-free or contained an excess of magnesium, conditions that inhibit secretion. Furthermore, although acetylcholine had some depolarizing effect in sodium-free media, the level to which the membrane potential fell was not below the control ;resting' potential since the cells in sodium-free medium were hyperpolarized; yet, secretory responses are augmented in such conditions.6. It is proposed that depolarization in response to acetylcholine may be no more than the electrical sign of increased permeability to ions such as sodium and calcium, and that depolarization is not, in itself, a key event in stimulus-secretion coupling. The evidence is held to favour the view that movement of calcium into the chromaffin cells on exposure to acetylcholine is responsible for evoking secretion.