When iontophoretic injections of horseradish peroxidase were made in the vicinity of inferior colliculus units in the cat responding to low sound frequencies, retrograde labelling occurred in the ipsilateral medial superior olive. Both bipolar and multipolar cells of the medial superior olive participated in this projection, and the focus of labelling shifted topographically within the olive as the injection site best frequency varied. These observations confirm previous anatomical findings and link them to the tonotopic organization of the central nucleus of the inferior colliculus. However, the fact that the medial superior olive alone provides between 50 and 98% of labelled cells in the brain stem projecting to this region of the central nucleus is an unexpected observation. This study gives further support to an hypothesis of 'core zones' within the central nucleus that receive preferentially input from specific brain stem auditory nuclei.