Single smooth muscle cells of rabbit intrapulmonary artery were voltage clamped using the perforated-patch configuration of the patch-clamp technique. We observed spontaneous transient outward currents (STOCs) and a steady-state outward current. Because STOCs were tetraethylammonium sensitive and activated by Ca2+ influx, they were believed to represent activation of Ca2+-activated K+ channels. The steady-state outward current, which was sensitive to 4-aminopyridine, was the delayed rectifier K+ current. In cells voltage clamped at 0 mV, we found that STOCs were not randomly distributed in amplitude but were composed of multiples of 1.57 +/- 0.56 pA/pF. The mean frequency of STOCs was 5.51 +/- 3.49 Hz. Ryanodine (10 microM), caffeine (5 mM), thapsigargin (200 nM), and hypoxia (PO2 = 10 mmHg) decreased STOCs. The effect of hypoxia on STOCs was partially reversible only if the experiment was conducted in the presence of thapsigargin. Hypoxia and thapsigargin decrease steady-state outward current. Thapsigargin and removal of external Ca2+ abolished the effect of hypoxia, suggesting that hypoxia decreases steady-state outward current by a Ca2+-dependent mechanism.