Membrane potential has been recorded in Y-1 adrenocortical cells with intracellular microelectrodes. Resting potential averaged -68 +/- 13 mV (n = 63). Cells were silent at rest but depolarization by current pulses evoked repetitive action potentials of 80-120 mV amplitude and variable duration (between 10-300 ms). Action potentials were unaffected by removal of external Na or addition of TTX, however they were completely abolished by substitution of external Ca by Co. In Ca-free solutions with 2.4 mM Ba, action potentials had a lower threshold and lasted for the whole duration of the current pulse.