We have previously used single channel patch-clamp and RT-PCR to show that a number of channels are present in both MG63 and SaOS2 osteoblast-like cells. Large conductance Ca²⁺-activated K⁺channels are amongst the most prevalent channels observed in these cells. The precise subunit composition of the native BK channel and its role remain however unknown. BK channels normally exist as a tetramer of 4 α subunits associated with β subunits. The subunit composition, specifically the type of β subunit co-assembled with the α subunits, modifies both the voltage- and Ca²⁺-sensitivity and the pharmacological fingerprint (e.g. sensitivity to iberiotoxin) of BK channels ¹ . Here we attempt a preliminary pharmacology of the channel, test its role in proliferation and try to specify the subunit composition of BK channels in MG63 cells. Single channel activity in osteoblast-like MG63 cells was measured in cell-attached, excised outside-out and inside-out patches. Cell numbers were measured by the MTS assay. RNA extraction, followed by RT-PCR was carried out to investigate the expression of α(KCNMA) and β (KCNMB) subunits of the BK channels. BK channels were readily observed in cell-attached patches at the resting membrane potential. Channel activity was voltage-dependent, activity increasing markedly on depolarisation, there often being up to 7 channels in some patches. External or internal tetraethylammonium chloride (TEA) reduced reversibly channel activity in excised patches. For example, in outside-out patches bath-application of 500 μM TEA typically reduced the open probability, producing an apparent reduction in unitary current, due to flickery channel block. More BK-selective blockers such as iberiotoxin (IbTX- 5-60 nM) and tetrandrine (5-30 μM) also produced reversible block of the channel in excised outside-out patches. Internal paxilline (10 μM) blocked reversibly the channel in excised inside-out patches. The channel was activated by external isopimaric acid (10 μM). RT-PCR showed that the α(KCNMA) subunit and the β1, β2, β3 and β4 (KCNMB1-4) subunits were present. Interestingly, TEA and tetrandrine had a dual effect on cell numbers. Treatment for 72 hours with TEA and tetrandrine increased significantly cell numbers at low concentrations (1 mM and 3 μM respectively), and reduced cell numbers at high concentrations (>10 mM and >10 μM). IbTX (10 -300 nM) did not alter cell numbers. Given these data suggest a possible role for BK channels in MG63 cell growth, these channels may therefore represent a target for therapeutic intervention. Future work aims to characterise fully the pharmacology of the BK channel, to define the native subunit composition, and the role of the channel in mineralization and secretion in osteoblast-like cells.