The place-frequency map of the developing rat cochlea was measured by iontophoretic HRP-injections into the ventral cochlear nucleus at electrophysiologically characterized positions. Distribution of retrograde HRP transport in cochlear spiral ganglion cells was analysed by means of a three dimensional reconstruction of the cochlea. Cochlear place-frequency maps were derived in rats of two ages groups: 13 to 22 days, and 36/37 day old animals. These maps were compared with the place-frequency map of adult rats (Müller, 1991). No systematic difference in the place-frequency map between 36/37 day old and adult rats was observed. In animals of the younger age group the place-frequency map (for frequencies above 4 kHz) was shifted towards lower frequencies for a given place along the basilar membrane. The morphological and physiological basis for this frequency shift during development is discussed.