The voltage clamp technique was used to study the effects of dendrotoxin (DTX) on outward potassium currents in internally perfused dorsal root ganglion neurones of guinea-pig. Sodium currents were eliminated by tetrodotoxin (TTX, 2 mumol/l), calcium currents and calcium-activated potassium conductances were abolished by intracellular perfusion of cells with KF. Depolarizing voltage shifts from a holding potential of -90 mV yielded a fast transient outward current (IfK) and a delayed non-inactivating outward current (IsK). These currents could be separated by shifting the membrane potential to -50 mV, where IfK was almost completely inactivated. DTX, at concentrations of 0.14-1.4 nmol/l selectively reduced a portion of the non-inactivating potassium current, leaving the transient outward current unaffected. Once manifested, the action of DTX could not be reversed by washing. The I-V characteristic of the current blocked by DTX is almost linear and quite different from the one of the 'DTX-resistant' portion of IsK, which shows a non-linear I-V curve. Tetraethylammonium (TEA, 30 mmol/l) strongly reduced IfK and IsK. However, subsequent application of DTX was still able to further reduce IsK. 3,4-diaminopyridine (3,4-DAP, 500 mumol/l) unselectively reduced IfK and a portion of IsK. The remainder of the latter could not further be reduced by DTX, suggesting a similar action of the two blockers on non-inactivating potassium currents. From the results presented, it is suggested that dendrotoxin selectively blocks a non-inactivating subtype of potassium channel.