The distribution of glycine-immunopositive cells and axonal endings was studied in the adult and early postnatal ferret superior olive. As in other species, the most prominent glycine-immunopositive cell group in the adult ferret superior olive was the medial nucleus of the trapezoid body. Other darkly immunostained cells were present, although more scattered, in most periolivary regions, including the lateral and ventral trapezoid body nuclei. In the lateral superior olivary nuclei, glycine-immunopositive cells were intermingled with immunonegative cells. A comparable population of cells in the ipsilateral lateral superior olivary nucleus was retrogradely labeled in cases with unilateral injections of tritiated glycine in the inferior colliculus. Glycine-immunopositive puncta were widely distributed in the neuropil in most periolivary regions, including dense accumulations in the dorsomedial periolivary region and ventral and lateral nuclei of the trapezoid body. In the lateral and medial superior olivary nuclei, immunopositive puncta were distributed around the principal cells in characteristic perisomatic halos. In postnatal ferrets, immunopositive cell bodies were first observed by postnatal day 7 and were distributed in regions comparable to regions in the adult, with the exception that immunopositive cells in the lateral superior olivary nucleus did not appear until about postnatal day 28. There was diffuse staining in the neuropil in principal and periolivary nuclei by postnatal day 7. During the third postnatal week, the immunostaining in the neuropil began to take on a more granular appearance and immunopositive puncta could be seen by postnatal day 35. In the lateral and medial superior olivary nuclei, the earliest distribution of immunostaining in the neuropil was nonuniform, being greater in the high-frequency, medial, and ventral regions, respectively. The density gradient in these areas was gradually eliminated over the next 2 postnatal weeks as immunostained processes and endings appeared over greater portions of the nuclei.