Rate-versus-level functions (RLFs) were recorded from individual cochlear nerve fibres in anaesthetised guinea-pigs. Variations in the shapes of these functions with frequency were used to derive input-output (IO) relationships for the mechanical preprocessing mechanisms in the cochlea. It was assumed that these preprocessing mechanisms operated linearly at frequencies well below each fibre's characteristic frequency (CF). The IO functions derived at each fibre's CF provided strong evidence of compressively nonlinear preprocessing in most regions of the cochlea. However, the apparent degree of compression depended on the fibre's CF, and hence on the presumed site of cochlear innervation. For fibres with CFs of between 1.5 and 3.6 kHz, the CF derived IO functions grew at rates of around 0.5 dB/dB. For fibres with CFs above 4 kHz, the IO functions were more compressive, with high-intensity asymptotic slopes of around 0.13 dB/dB. In the highest (> or = 10 kHz) CF fibres, the degree of compression depended on the physiological condition of the cochlea; the derived IO functions becoming more linear as the cochlea became less sensitive. The derived IO technique was not well suited to analyse responses evoked by very low frequency (e.g., < 500 Hz) tones. Nonetheless, the CF RLFs from fibres with CFs lower than approximately 1 kHz provided little evidence of mechanical nonlinearity near the apex of the cochlea. These findings imply a longitudinal variation in the mechanisms of cochlear preprocessing, and provide important new tests for functional models of the cochlea.