Positive deflection of the sensory hair bundle of a vertebrate hair cell opens transduction channels to depolarize the cell. In bullfrog saccular hair cells, there is a subsequent adaptation process, whereby the proportion of transduction channels that are open, and thus the receptor current, declines toward the resting value. This occurs because the sensitivity curve, relating open probability to bundle deflection, shifts along the deflection axis in response to bundle deflections, in a manner consistent with a relaxation of mechanical tension on transduction channels. In this study we determined the extent of adaptation, measured as the shift of the sensitivity curve following deflection of the hair bundle. The shift was determined both by comparison of the receptor current in the adapted state to the resting sensitivity curve, and by comparison of pre- and postadapted sensitivity curves. The adaptive shift approached steady state with a time constant of 20–30 msec, and was at steady state within 150 msec. For all positive and for small negative deflections, both methods showed a shift that was approximately 80% of the deflection. For larger negative deflections, the shift reached a fixed limit that was 100–500 nm negative to the freestanding bundle position. The limited extent of adaptation confers a time-dependent sensitivity: the cell has an instantaneous or phasic sensitivity curve that is steep, and steady- state or tonic sensitivity curve that is about five times broader. It also suggests the existence of two additional structural elements within the transduction apparatus. A revised quantitative theory accommodates these elements.