The first step towards the generation of the receptor potential in hair cells is the gating of the transducer channels and subsequent flow of transducer current, induced by deflection of the stereocilia. We describe properties of the transducer current in outer hair cells of neonatal mice. Less extensive observations on inner hair cells suggest that their transducer currents have similar characteristics. The hair bundles were stimulated by force from a fluid jet. The transducer currents in outer hair cells are the largest found so far in any hair cell, with a chord conductance of up to 9.2 nS at -84 mV. The transfer function suggests that the channel has at least two closed states and one open state. The permeabilities for sodium, potassium and caesium are similar, consistent with the channel being a fairly non-selective cation channel. At negative potentials the currents adapt in most cells, although never as completely as in hair cells of lower vertebrates. If the unit conductance of the transducer channel is similar to that of the turtle's auditory hair cells (100 pS), then there are about 90 channels per hair bundle, or one channel between every pair of adjacent stereocilia in neighbouring rows.