The effects of an anti-insect scorpion alpha toxin, LqhαIT, isolated from the venom of the Israeli scorpion Leiurus quinquestriatus hebraeus was studied on isolated adult dorsal unpaired median (DUM) neurones isolated from the cockroach Periplaneta americana terminal abdominal ganglion. Using the cell-attached patch-clamp configuration, a new type of sodium channel, called background sodium channel (bNa), was recently characterized. At -50mV, the channel activity was observed as unclustered brief single openings. For hyperpolarized steady-state holding potential (-100mV) the patches contained large unitary current steps, appearing generally in bursts. The open probability (Po) calculated at -50mV was low (0.008 (±0.004), n =5) and displayed a typical bell-shaped voltage dependence. LqhαIT (10 -8 M) altered the bNa activity in a time-dependent manner. At -50mV the channel activity appeared in bursts. Po calculated at -50mV was about 20 times greater than Po calculated in controls and also showed bell-shaped voltage dependence. At 10 -7 M, LqhαIT induced longer silent periods interrupted by bursts of increased channel activity. Whole-cell recordings revealed that 10 -7 M LqhαIT transformed regular beating DUM neurone pacemaker activity into a rhythmic bursting. In this paper we demonstrate, for the first time, that bNa is a new target for anti-insect scorpion toxin.