Aims: BK channels play a key role as negative feedback regulators of cell excitability. In smooth muscle, PKA and PKG activate BK channels by increasing the apparent voltage- and Ca²⁺-sensitivity of the channel, leading to smooth muscle relaxation. In contrast, PKC inhibits BK channel activity und thus reinforces contraction. Although BK channel inhibition by PKC has physiological and pathophysiological relevance, its molecular mechanism has remained elusive and was therefore analysed in this study. Methods: Site-directed mutagenesis was carried out by PCR techniques. Patch-clamp recording techniques were used to measure currents from transfected HEK293 or tracheal smooth muscle cells in the whole-cell , outside-out- or inside-out patch-clamp configuration. Results: Screening of the BK channel for PKC consensus sequences reveals nine putative phosphorylation sites (S390, S499, S625, S630, S695, S712, S1129, S1151 and S1154). Only two of the tested mutants (S695A and S1151A) were insensitive to PKC-induced BK channel inhibition. The phosphomimetic mutant S1151D did not significantly differ from WT channels, whereas the S695D mutant resulted in channels with strongly reduced membrane conductance (47.4 ±2.1 % of the WT). PKG application to the S1151D mutant resulted in a 26 mV shift to the left of the conductance- voltage relation, whereas PKA had no significant influence. In contrast, S1151A mutants proved insensitive to stimulation by PKG but were responsive to PKA. The S695D mutant was completely insensitive to either PKG or PKA. Conclusions: Our data show a very dynamic regulation of the BK channel by PKC activity. Phosphorylation/dephosphorylation of S1151 determines BK channel sensitivity towards PKG and PKA. Phosphorylation of S695 which is conditional, depending on phosphorylation of Ser1151, stabilizes the nonconducting conformation of the channel and excludes activation by PKG and PKA.