The effects of insect toxin Bot IT 2 , purified from the venom of the scorpion Buthus occitanus tunetanus, were investigated on the isolated giant axon and on isolated dorsal unpaired median (DUM) neurone of the cockroach Periplaneta americana under current- and voltage-clamp conditions, using the double-oil-gap technique and the patch-clamp technique, respectively. In both preparations, Bot IT 2 induces a limited depolarization together with the development of a repetitive activity in axon and an increase of spontaneous discharge frequency in DUM neurone. After artificial hyperpolarization to the normal resting level, plateau potentials can be evoked in both preparations. Under voltage-clamp conditions, Bot IT 2 induces a similar effect in axon and DUM neurone by acting specifically on sodium channels. The peak sodium current is decreased and simultaneously a new current with very slow activation-deactivation kinetics is developed. Voltage dependence of this slow current is not very different from that of the control. The inactivation of the fast component is incomplete because it is masked by the development of the slow component. These results suggest that Bot IT 2 modifies the kinetics of insect sodium channel activation, and the transformation of normal fast channels into slow ones is discussed.