Abstract The purpose of the present study was to describe, at the single-channel level, the activity of a calcium-sensitive potassium channel in rat visceral-sensory neurons which has been suggested to be involved in sensory neuron excitability. Single-channel recordings in the inside-out configuration identified a 220 pS conductance calcium-activated potassium channel (KCa). From a −20 mV holding potential, increasing [Ca 2+] i from 0.01 μM to 1.0 μM increased the open probability of this channel 92% (from 0.12 to 0.23). However, from a +20 mV holding potential, increasing [Ca 2+] i from 0.01 to 1.0 μM increased the open probability by 326% (from 0.15 to 0.64). In addition, this large conductance KCa channel was blocked by TEA (1.0 μM) and charybdotoxin (40 μM) when applied to the external surface. These results are the first to characterize a large conductance KCa channel in the sensory afferent neurons of the rat nodose ganglia and should further expand the understanding to the ionic currents involved in the regulation of sensory afferent neuronal activity.