Slow potentials generated in the sensory organ of the ampulla were recorded in isolated semicircular canals of the frog by means of fluid electrodes. These potentials, which may be picked up from the intrampullar fluid and those from the ampullar nerve appear to be generated at different stages of the process taking place in crista ampullaris. Slow intra-ampullar potentials apparently reflect receptor potentials of hair cells. They are preserved after degeneration of the nerve fibre endings and are relatively insensitive to DNP poisoning; their amplitude is maximum at high K+ concentrations. Slow nerve potentials appear to be due to electronic spreading of post-synaptic excitatory potentials generated at cytoneural junctions. They disappear after degeneration of the nerve fibres, in low Ca++ and high Mg++ solutions and are extremely sensitive to DNP poisoning. An analysis of the time-course of the slow ampullar and nerve potentials referred to the discharge of impulses in afferent fibres was performed with a view to interpreting the transduction mechanism of semicircular canals.