The mechanism underlying the hyperpolarization induced by isoprenaline in mouse lumbrical muscle fibres was studied using cell-attached patch and intracellular membrane potential (V _m) recordings. Sarcolemmal inwardly rectifying K⁺ channels (K_IR: 45 pS) and Ca²⁺-activated K⁺ channels (BK: 181 pS) were identified. Exposure to isoprenaline closed K_IR channels and increased BK channel activity. This increase was observed as a shift from 50 to −40 mV in the voltage dependence of channel activation. Isoprenaline prevented hysteresis of V _m when the extracellular [K⁺] fell below 3.8 mM. This hysteresis was due to the properties of the K_IR. The effects of chloride transport and isoprenaline on V _m did not interact purely competitively, but isoprenaline could prevent the depolarization induced by hyperosmotic media equally as well as bumetanide, which inhibits the Na⁺/K⁺/2Cl⁻ cotransporter. In lumbrical muscle this leads to hyperpolarization, but this might vary among muscles. The switch from K_IR to BK as the component of total K⁺ conductance was due to isoprenaline.