The generator potential in sensory afferents of frog crista ampullaris was extracellularly recorded from the cut end of the posterior ampullary nerve by means of suction electrodes. A servocontrolled turntable allowed suitable rotatary stimulations. The analysis of the recorded generator potential revealed a different time course from that predicted on the basis of the pendulum model. Adaptation and undershoots in the responses to velocity ramps, steps and sinusoids, were mainly responsible for the deviations, which became very evident only when fairly high acceleration rates were applied. Both adaptation and undershoots were produced presumably by the activation of an electrogenic pump, probably located in nerv terminals contacting the hair cells. In fact, the time course of the generator potential became much more consistent with the predictions from the pendulum model under treatments capable of hindering the ion pump activity.