1. Recordings of extracellular activity were obtained from single cells in the spiral ganglion of the basal turn of the guinea pig cochlea. 2. The spatial distribution of characteristic frequencies of cells in the ganglion was consistent with published data on the location of displacement maxima on the basilar membrane. 3. Large variations in the sharpness of single cell tuning curves were seen between animals. These variations were closely linked to sensitivity differences. 4. The tuning curves of single cells could be made less sharp by slowing the rate of artificial ventilation. These tuning curve changes were reversible and intimately associated with alterations in sensitivity and spontaneous activity. 5. The data point either to the presence of a mechanical non-linearity, or a physiologically vulnerable second filter, as the explanation for the sharpness of neural tuning curves in cochlear nerve fibres.