The whole-cell voltage-clamp method was applied to single smooth muscle cells prepared from the longitudinal layer of the pregnant rat myometrium (17-20 days of gestation). It was found that the transient inward current mainly consists of Ca 2+ current, because the removal of Ca 2 Β§ ions from the external medium and 10~M nifedipine eliminated this inward current. Its steady-state inactivation curve was obtained by the standard method, in which the membrane potential of half inactivation and the slope factor were estimated to be -58.0+_4.9mV (n= 11) and 8.9+1.4mV (n= 11), respectively. In a small number of preparations (in 2 out of 30 preparations), there remained a very fast inward current in CaZ+-free medium containing Mg 2+ . Tetrodotoxin (TTX, 10 IxM) can abolish this current, suggesting that the channel for this current is equivalent to the Na + channel in nerve cells. Two major phases of outward currents were identified by voltage jumps from negative holding levels to more positive levels. The first phase was a fast transient outward current. This current remained intact after external tetraethylammonium (TEA, 20 mM) was added. Following the transient current, a large delayed rectified outward current reached its peak over a period of 50 ms and then decayed. The reversal potential for this outward current was determined by observing the change of polarity of the tail currents with the change in extracellular K + concentration ([K+]o). The slope for the change of reversal potential per ten-fold change in [K+]0 is 57.7 mV at more than 23.2 mM [K Β§ indicating that this current is mostly carried by K + ions. Voltage-dependent inactivation of the delayed rectified outward current was determined by the standard method. The membrane potential for half inactivation and the slope factor were estimated to be -42.8 + 3.9 mV (n = 3) and 10.1_l.5mV (n = 3), respectively. External TEA (20 mM) effectively eliminated the delayed rectified outward currents. Nifedipine (10 gM) suppressed not only Ca 2+ current but also outward K+currents.