A comparative study was carried out on the inactivation of Na⁺ channels in two types of endocrine cells in rats, -cells and adrenal chromaffin cells (ACCs), using patch-clamp techniques. The -cells were very sensitive to hyperpolarization; the Na⁺ currents increased ninefold when the holding potential was shifted from -70 mV to -120 mV. ACCs were not sensitive to hyperpolarization. The half-inactivation voltages were -90 mV (rat -cells) and -62 mV (ACCs). The time constant for recovery from inactivation at -70 mV was 10.5 times slower in -cells (60 ms) than in ACCs (5.7 ms). The rate of Na⁺-channel inactivation at physiological resting potential was more than three times slower in -cells than in ACCs. Na⁺ influx through Na⁺ channels had no effect on the secretory machinery in rat -cells. However, these 'silent Na⁺ channels' could contribute to the generation of action potentials in some conditions, such as when the cell is hyperpolarized. It is concluded that the fractional availability of Na⁺ channels in -cells at a holding potential of -70 mV is about 15 % of that in ACCs. This value in rat -cells is larger than that observed in mouse (0 %), but is smaller than those observed in human or dog (90 %).