In chromaffin cells, plasma membrane calcium (Ca²⁺) channels and mitochondria constitute defined functional units controlling the availability of Ca²⁺ nearby exocytotic sites. We show here that, when L‐/N‐type Ca²⁺ channels were inhibited with nisoldipine and ω‐conotoxin GVIA, cytosolic [Ca²⁺] ([Ca²⁺]_c) peaks measured in fura‐4F‐loaded cells were reduced by 36%; however, mitochondrial Ca²⁺ uptake was unaffected and secretion was potentiated by protonophores as in control cells. By contrast, when non L‐type Ca²⁺ channels were inhibited with ω‐conotoxin MVIIC, [Ca²⁺]_c peaks induced by high K⁺ were reduced by 73%, mitochondrial Ca²⁺ uptake was abolished, and secretion was not modified by protonophores. However, if Ca²⁺ entered only through L‐type channels activated by FPL64176, high K⁺ stimulation induced fast mitochondrial Ca²⁺ uptake and catecholamine secretion was strongly increased and potentiated by protonophores. These results confirm the close association of catecholamine secretion to mitochondrial Ca²⁺ uptake, and indicate the sharp threshold of local [Ca²⁺]_c (about 5 µm) required for triggering fast mitochondrial Ca²⁺ uptake that is able to modulate secretion. The entry of Ca²⁺ through L‐type channels generated local [Ca²⁺]_c increases just below that, inducing little mitochondrial Ca²⁺ uptake unless FPL64176 was present. By contrast, Ca²⁺ entry through P/Q‐type channels fully activated mitochondrial Ca²⁺ uptake. Control of secretion by mitochondria therefore depends critically on the ability of the stimulus to create large local [Ca²⁺]_c microdomains.