The muscarinic-regulated potassium current (M-current), formed by the heteromeric assembly of subunits encoded by the KCNQ2 and KCNQ3 genes, is a primary regulator of neuronal excitability; this regulation is accomplished by impeding repetitive firing and causing spike-frequency adaptation. Mutations in KCNQ2 or KCNQ3 cause benign familial neonatal convulsions (BFNC), a rare autosomal-dominant generalized epilepsy of newborns, by reducing the maximal current carried by the M-channels without affecting ion selectivity or gating properties. Here we show that KCNQ2/KCNQ3 channels carrying a novel BFNC-causing mutation leading to an arginine to tryptophan substitution in the voltage-sensing S4 domain of KCNQ2 subunits (R214W) displayed slower opening and faster closing kinetics and a decreased voltage sensitivity with no concomitant changes in maximal current or plasma membrane expression. These results suggest that mutation-induced gating alterations of the M-current may cause epilepsy in neonates.