Abstract 1. L-Glutamate, L-aspartate, kainate, but not N-methyl- D-aspartate, increased the spontaneous activity recorded from the afferent nerve fibers of isolated lateral line hair cell stitches of Xenopus laevis. 2. Concentrations of glutamate applied to the serosal surface of adult animals were less than those necessary to evoke a response applying to the cupular surface and similar to those used in young Xenopus and in the guinea-pig cochlea. 3. Higher concentrations of the active substances applied to the lateral line resulted in greater excitation followed by suppression of spontaneous activity. 4. The results are consistent with the hypothesis that L-glutamate or aglutamate-like compound may be the afferent transmitter released by the hair cells of lateral line and guinea-pig cochlea.