The effects of an anti-insect scorpion alpha toxin, LqhaIT, 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 con®guration, a new type of sodium channel, called background sodium channel (bNa), was recently characterized. At À50 mV, the channel activity was observed as unclustered brief single openings. For hyperpolarized steady-state holding potential (À100 mV) the patches contained large unitary current steps, appearing generally in bursts. The open probability (Po) calculated at À50 mV was low (0.008 (AE 0.004), n = 5) and displayed a typical bellshaped voltage dependence. LqhaIT (10 À8 M) altered the bNa activity in a time-dependent manner. At À50 mV the channel activity appeared in bursts. Po calculated at À50 mV was about 20 times greater than Po calculated in controls and also showed bell-shaped voltage dependence. At 10 À7 M, LqhaIT induced longer silent periods interrupted by bursts of increased channel activity. Whole-cell recordings revealed that 10 À7 M LqhaIT transformed regular beating DUM neurone pacemaker activity into a rhythmic bursting. In this paper we demonstrate, for the ®rst time, that bNa is a new target for anti-insect scorpion toxin.