Large conductance, Ca2+-activated K+ (maxi-K ) channel activity was recorded in excised, inside-out patches from HEK 293 cells stably co-expressing the a- and ß-subunits of human brain maxi-K channels. At +50 mV, and in the presence of 300 nM Ca2+i, single channel activity was acutely and reversibly suppressed upon reducing PO2 from 150 to > 40 mmHg by over 30 %. The hypoxia-evoked reduction in current was due predominantly to suppression in NPo, although a minor component was attributable to reduced unitary conductance of 8-12 %. Hypoxia caused an approximate doubling of the time constant for activation but was without effect on deactivation. At lower levels of Ca2+i(30 and 100 nM), hypoxic inhibition did not reach significance. In contrast, 300 nM and 1 µM Ca2+i both sustained significant hypoxic suppression of activity over the entire activating voltage range. At these two Ca2+i levels, hypoxia evoked a positive shift in the activating voltage (by approximately 10 mV at 300 nM and approximately 25 mV at 1 microM). At saturating [Ca2+]i (100 µM), hypoxic inhibition was absent. Distinguishing between hypoxia-evoked changes in voltage- and/or Ca2+i-sensitivity was achieved by evoking maximal channel activity using high depolarising potentials (up to +200 mV) in the presence of 300 nM or 100 µM Ca2+i or in its virtual absence (> 1 nM). Under these experimental conditions, hypoxia caused significant channel inhibition only in the presence of 300 nM Ca2+i. Thus, since regulation was observed in excised patches, maxi-K channel inhibition by hypoxia does not require soluble intracellular components and, mechanistically, is voltage independent and Ca2+i sensitive.