Type I vestibular hair cells isolated from guinea pig were placed in the whole cell clamp configuration, and electrically stimulated by depolarizing voltage pulses. The voltage dependent reversible movements of the cell apex affected the length of the cell neck, the position of the cuticular plate, and the tilting and bending of the stereocilia. The cell neck shortened when the membrane was depolarized by 10 mV while cuticular plate and the stereocilia tilting did not begin until 20 mV. The shortening was 0.5 to 1 micron, and the cuticular plate tilting was up to 15 degrees for depolarization amplitudes of 20-40 mV. These movements were reversed within a few seconds. More complex, larger movements were induced by stronger depolarizations. The cuticular plate tilting and the hair bundle bending were always in the opposite direction to the kinocilium position. The small reversible movements of the mammalian type I vestibular hair cells are discussed in terms of mechanical adaptation processes and morphological features. It is suggested that such active movements of the vestibular hair cells occur in vivo.