Effects of Na, Ca, anions and isoprenaline on the plateau potential in circular muscle of 11-to 13-day pregnant rat myometrium were investigated using the double sucrose gap method. During the plateau potential, which amounted about 15-20mV and lasted for 10-20 sec, the membrane conductance increased initially, then decreased. The amplitude and duration of the plateau potential increased by raising the external Ca concentration to between 0.3 and 3mM, and decreased when the concentration was increased more. The plateau was prolonged in low Na solution (54mM). When 154mM Cl was replaced with Br the plateau was prolonged, however, the plateau was nearly suppressed when Cl was replaced with benzene sulphonate or isethionate. Isoprenaline increased membrane conductance and depressed the plateau. It was concluded that a decrease in K conductance was largely responsible for the plateau potential, and an inward Ca current may have contributed to its formation.