Single channel currents were recorded from uterine smooth muscle cells of pregnant guinea-pigs. The cells were within small bundles of smooth muscle of low input resistance. Giga-ohm seals were formed between the patch clamp electrodes and the membrane of smooth muscle cells that had not been treated with enzymes. In cell-attached mode, outward current steps were observed that had a conductance of 130-170 pS, a reversal potential close to the potassium reversal potential, and the probability of a channel being in the open state increased e-fold per 8-11 mV depolarization. In the cell-free, inside-out patch mode, currents were recorded that had a large conductance, 420 pS, and smaller conductance levels. When the chloride concentration was changed the reversal potential shifted in a manner consistent with the Nernst equation indicating that the channels were permeable to chloride ions.