Mammalian vestibular organs have two types of sensory cells: Type I and Type II hair cells. To compare signalling properties of these cell types, we study them in vitro, where their voltage responses (receptor potentials) to controlled manipulations of their hair bundles can be examined. The guinea pig utricle is excised and maintained in medium; accessory structures are removed to expose the hair bundles of the sensory cells. A fine probe moves the hair bundle of an individual cell while a microelectrode inserted in the cell records its voltage. Although normal resting potentials have been recorded, we have not yet detected responses to the hair bundle motion.