K+ channels were studied in oligodendrocytes in cultures of mouse spinal cord. Single channel currents were measured using the gigaseal technique. The conductance of the channels varied greatly i.e. from 6 to 125 pS (38 +/- 28 SD, N = 21). In some patches there were up to three current levels of the same size. At -70 mV the open state probability was 0.51 +/- 0.17 and the average duration of an opening 70 +/- 20 ms for 4 channels with conductance from 16-57 pS. These analyses exclude brief flickering (less than 2 ms) or long closed periods (seconds to minutes). These times were not markedly affected by pulling the patch off the cell or by superfusing the isolated patch with media containing 10 mmol X 1(-1) TEA or EGTA without Ca2+. At membrane potentials between -90 and -30 mV there was a small but consistent effect of depolarization to increase the open state probability. Large positive or negative voltage steps decreased the open state probability. Current voltage measurements on intact cells showed a striking decrease in membrane conductance at these large membrane potentials. The leakage conductance of the patch also exhibited some K+ selectivity. The oligodendrocyte membrane appears to contain about one K+ channel per 5 micron 2. The known electrical properties of cultured oligodendrocytes can essentially be explained by the distribution and properties of these K+ channels.