In primary sensory cells, a K current active at resting potential is selectively blocked by the convulsant snake toxin, dendrotoxin. Using the patch-clamp technique, we have examined the characteristics of this K current at the unitary level. The voltage-activated K+ channels were found to have a maximum conductance of 5-10 pS in a 'physiological' K+ gradient. They show negligible sensitivity to calcium at the inner membrane aspect. Blockade by dendrotoxin seems likely to be due to direct action on the K+ channel.