Retrograde transport of horseradish peroxidase using tetramethylbenzidine as a chromagen was used to map the cell bodies of origin of the efferent projection to the cochlea in the guinea pig. Large numbers of small labelled neurones were found within the body of the lateral superior olivary nucleus (LSO) ipsi-lateral to the injected cochlea. Labelled cells within the contra-lateral LSO were extremely rare (1% or less). Large labelled neurones were found both ipsi-laterally and contra-laterally in the dorsomedial periolivary region, including the medial nucleus of trapezoid body and in the ventral and lateral nuclei of the trapezoid body and the ventral nucleus of lateral lemniscus. It was concluded that some aspects of the distribution of these efferent neurones in the guinea pig are similar to the situation in the rat, whilst others resemble more closely the arrangement found in cat.