Recordings were obtained from single primary auditory neurones in the guinea pig cochlea during perfusion of the scala tympani with solutions containing elevated concentrations of Mg2+, Mn2+ and Co2+, or with lowered concentrations of Ca2+. Such perfusions caused a reversible depression of spontaneous firing rates. This is consistent with the notion that spontaneous firing is the result of background release of excitatory transmitter from cochlear hair cells, in the absence of acoustic stimulation. The above ion modifications also produced varying changes in single neurone response curves to acoustic stimuli. In one half of the 14 neurons studied these changes were also compatible with a classical blockage of synaptic transmission. The other half however, showed little or no change in sensitivity at low frequencies while large threshold elevations occurred at high frequencies.