Dexmedetomidine (DEX), a selective agonist of α₂-adrenergic receptors, is recognized to facilitate analgesia and anaesthesia in humans. Despite the potential for wide use, its effects on ion currents and membrane potential in neurones remain largely unclear. We investigated the effects of DEX on ion channels in NG108-15 neuronal cells differentiated with dibutyryl cyclic AMP and in cultured cerebellar neurones. DEX suppressed the amplitude of delayed rectifier K⁺ current [I_K(DR)] in a concentration-dependent manner with an IC₅₀ value of 4.6 µM in NG108-15 cells. No change in the steady-state inactivation of I_K(DR) was evident in the presence of DEX. A minimal binding scheme was also used to evaluate DEX-induced block of I_K(DR). Inhibition of I_K(DR) by DEX was still observed in cells preincubated with yohimbine (10 µM) or efaroxan (10 µM). DEX depressed the peak amplitude of Na⁺ current (I_Na), whereas it had minimal effect on L-type Ca²⁺ current. Under current-clamp configuration, DEX increased the duration of action potentials (APs). I_K(DR) and I_Na in response to AP waveforms were more sensitive to block by DEX than those elicited during rectangular pulses. In isolated cerebellar granule cells, DEX also effectively suppressed I_K(DR). The effects of DEX are not limited to its interactions with α₂-adrenergic receptors. Inhibitory effects on I_K(DR) and I_Na constitute one of the underlying mechanisms through which DEX and its structurally related compounds might affect neuronal activity in vivo.