The aim of this study was to examine the effects of acute hypoxia on the slow (IKs) and rapid (IKr) components of the native delayed rectifier K+ channel in the absence and presence of the -adrenergic receptor agonist isoproterenol (Iso) using the whole- cell configuration of the patch-clamp technique. Hypoxia reversibly inhibited basal IKs. The effect could be mimicked by exposing the cells to the thiol-specific reducing agent dithiothreitol (DTT) and attenuated upon exposure of cells to the membrane impermeant thiol- specific oxidising compound 5,5'-dithio-bis[2- nitrobenzoic acid] (DTNB). In the presence of hypoxia, the K0.5 for activation of IKs by Iso was significantly decreased from 18.3 to 1.9 nM. DTT mimicked the effect of hypoxia on the sensitivity of IKs to Iso while DTNB had no effect. Hypoxia increased the sensitivity of IKs to histamine and forskolin suggesting that the effect of hypoxia is not occurring at the -adrenergic receptor. The increase in sensitivity of IKs to Iso could be attenuated with addition of PKC peptide to the pipette solution. While hypoxia and DTT inhibited basal IKs it was without effect on IKr. In addition, Iso did not appear to alter the magnitude of IKr in the absence or presence of hypoxia. These data suggest that hypoxia regulates native IKs through two distinct mechanisms: direct inhibition of basal IKs and an increase in sensitivity to Iso that occurs downstream from the -adrenergic receptor. Both mechanisms appear to involve redox modification of thiol groups. In contrast native IKr does not appear to be regulated by Iso, hypoxia or redox state