The expression of K_ATP channels by magnocellular cholinergic basal forebrain (BF) neurones was investigated in thin brain slice and dissociated cell culture preparations using a combination of whole- cell, perforated-patch and single-channel recording techniques. Greater than 95% of BF neurones expressed functional K_ATP channels whose activation resulted in membrane hyperpolarisation and a profound fall in excitability. The whole-cell K_ATP conductance was 14.0 ± 1.5 nS with a reversal potential of -91.4 ± 0.9 mV that shifted by 59.6 mV for a tenfold increase in [K⁺] _o. IK_ATP was inhibited reversibly by tolbutamide (IC₅₀ of 34.1 µM) and irreversible by glibenclamide (0.3-3 nM) and had a low affinity for [ATP]_i (67% reduction with 6 mM [MgATP]_i Using perforated patch recording a small proportion of the conductance was found to be tonically active. This was weakly potentiated by diazoxide (0.1 mM extracellular glucose) but insensitive to pinacidil ( 500 µM). Single channel K_ATP currents recorded in symmetrical 140 mM K⁺ - containing solutions exhibited weak inward rectification with a mean conductance of 66.2 ± 1.9 pS. Channel activity was inhibited by MgATP (>50 µM) and activated by MgADP (200 µM). The K⁺ channel opener diazoxide (200-500 µ M) increased NPo by 486 ± 120% whereas pinacidil (500 µM) had no effect. In conclusion, the characteristics of the K_ATP channels expressed by BF neurones are very similar to channels composed of SUR1/Kir6.2 subunits. In the native cell, their affinity for ATP is close to the resting [ATP]_i potentially allowing them to be modulated by physiologically relevant changes in [ATP]_i. The effect of these channels on the level of ascending cholinergic excitation of the cortex and hippocampus are discussed.