The superior olivary complex (SOC) of the adult rat brainstem was studied in detail with regard to its innervation by neural elements showing immunoreactivity for two neuroactive peptides, somatostatin and substance P. Nerve fibres and varicosities showing positive immuno-reactivity for both peptides were particularly dense immediately dorsal and lateral to the lateral superior olivary nucleus (LSO) and dorsal to the superior paraolivary nucleus (SPN). Penetration of this curtain-like innervation into the SPN was limited, and the LSO showed only a very minor innervation by somatostatin-positive structures in its most medial (high frequency) lobe. Dense fibre labelling and varicosities were also apparent for both peptides immediately medial to the ventral and dorsal nuclei of the lateral lemniscus, and in the external cortex and dorsomedial zones of the inferior colliculus (IC). Labelled fibres and endings were also seen in the granule cell regions of anteroventral cochlear nucleus (AVCN) and the most dorsomedial parts of the dorsal cochlear nucleus (DCN). The majority of cells in the medial nucleus of the trapezoid body (MNTB) showed a prominent innervation by nerve terminals that stained positive for somatostatin only whereas the medial superior olivary nucleus (MSG) was devoid of label for both peptides. The ventral nucleus of the trapezoid body (VNTB) showed sparse but significant innervation by both somatostatin and substance P-positive structures. Hence the VNTB was the only defined nucleus of the SOC to show a significant substance P-positive innervation. Neuronal somata immune-reactive for somatostatin were found in anteroventral and posteroventral cochlear nuclei (AVCN and PVCN) and the A5 and A7 cell groups adjacent to the LSO and the VNLL and DNLL and in all subdivisions of the inferior colliculus (IC). Somata showing only faint immunoreactivity for substance P were found in the VNLL, AVCN and PVCN. These results suggest a potential role for both peptides in auditory signal processing in the adult rat brain. (C) 1997 Elsevier Science B.V.