The fine structure of the Torpedo electric organ was investigated during different functional states using fixation media with elevated calcium concentrations. When electric tissue was stimulated (5,000 pulses, 5 Hz) and subsequently fixed in an s-collidine buffered aldehyde fixative containing 5 mM CaCl2 mitochondria in both nerve terminals and postsynaptic electroplaque cells showed electron-dense deposits. When tissue blocks were stimulated in the presence of d-tubocurarine (250 microM) which blocked electrical response deposits were restricted to presynaptic mitochondria. Electron-dense deposits were never observed in unstimulated tissue. The observations suggest that after stimulation mitochondria have an increased calcium binding capacity. In conjunction with previous biochemical findings (Schmidt and Zimmermann, 1980), we suggest that this increased binding capacity is due to a previous stimulation induced loss of calcium from activated mitochondria.