An upper bound on the magnitude of semicurcular canal cupula motion was experimentally determined in the isolated labyrinth of the skate, Raja erinacea. To visualize the cupula, a glass pipette was pushed through the ampullary wall, and local regions of the cupula were stained by slow infusion of small amounts of Alcian Blue dye. Caloric stimuli which produced large changes in single unit activity in the ampullary nerve and which often recruited several larger, previously silent units were found to produce on detectable cupula motion as seen through the ampulla wall. However when the cupula was first grossly displaced, motion was thereafter observed in response to identical caloric stimuli. Analysis of afferent responses indicates that the normal range of cupula motion in the skate is below the optical resolution of the method, conservatively estimated as 3--5 micrometers.