We sense the temperature of our skin and surroundings using specific thermoreceptors, which are sensitive to cold and warmth, but little is known about how these receptors transduce temperature into electrical activity. We have discovered an inward ionic current that is activated by moderate cooling in a small number of rat sensory neurons. This current has features that are found in intact cold receptors, including sensitization by menthol, adaptation upon sustained cooling, and modulation by calcium, and is likely to be important in cold sensing.