TxIA and TxIB, peptides with 27-amino acid residues recently isolated from the molluscivorous marine snail Conus textile neovicarius, exhibit strong paralytic activity in molluscs, with no paralytic effects on athropods and vertebrates. At concentrations of 0.25-0.5 microM the toxins cause spontaneous repetitive firing and dramatic broadening of the action potential of cultured Aplysia neurons. The action potential duration partially recovers within 30 min in the presence of the toxins. Under these conditions a second toxin application does not change the spike duration. TxI-induced spike broadening occurs when potassium and calcium conductances are blocked. Voltage-clamp experiments revealed that the toxins alter the kinetics of the sodium current either by slowing down the rate of sodium current inactivation or by recruiting silent sodium channels with slower activation and inactivation kinetics. The toxins shift the voltage-dependent steady-state Na+ current inactivation curve to more positive values by 6 mV. These changes are not associated with alteration in the rate of sodium current activation, in the peak sodium current, or the sodium current reversal potential. TxI apparently represents a new class of conotoxins with an unusual phylogenic specificity and may therefore be useful as a probe for the study of molluscan neuronal sodium channels.