In the mammalian auditory nerve physiological recordings revealed that the spontaneous discharge rate of single auditory fibres correlates with the diversity of input-output functions which may be important for intensity discrimination (e.g., Sachs and Abbas, 1974, Liberman, 1978;
Winter et al., 1990). In this study we determined if the spontaneous discharge rate of auditory nerve fibres in the guinea pig is correlated with an anatomical feature, namely the diameter of the respective fibres. The diameter of myelinated (Type I) guinea pig auditory nerve fibres was
measured after staining with different techniques. Measurements were made on semithin sections using a video image analysis system. The diameters of fibres stained with toluidine blue from the portion of the auditory nerve containing fibres from the basal turn of the cochlea were
found to have a normal distribution. Fibres were also labelled with horseradish peroxidase by bulk injection into the spiral ganglion. It was found that the presence of horseradish peroxidase within the fibres reduced the measured diameter in comparison to adjacent unlabelled fibres. A number of fibres were physiologically characterized with respect to spontaneous discharge rate and subsequently intracellularly labelled with horseradish peroxidase. Fibre diameter of a selected sample of intracellularly labelled fibres was measured over a distance of 800 pm within the internal auditory meatus. At the positions nearest to the spiral ganglion fibres possessing low spontaneous rates were found to have smaller diameters than high spontaneous rate fibres. No difference in fibre diameter was found for the positions near the cochlear nucleus.