The development of the size and tonotopy of the mouse inferior colliculus (IC) was studied at postnatal ages of 9-20 days. During that time, the size of the IC remained constant in all 3 planes (rostrocaudal, mediolateral dorsoventral). At day 10, the first low-frequency responses without tonotopy could be recorded from neurons in the rostral and central parts of the central nucleus sparing its caudal part, very medial portions, the medial part (M) of the central nucleus, the dorsal cortex and the lateral nucleus. Then, an extension of the frequency responsiveness occurred towards (1) the caudal pole which was reached by about day 14, (2) the dorsal surface reached between days 12 and 14, (3) the ventral border of the IC reached by about day 15. The high-frequency nucleus of the IC (M part of the central nucleus) remained unresponsive to tones up to day 13. Between days 10 and 20, there was a constant increase of highest characteristic frequencies (CFs) measurable of neurons in the IC. During that time, lowest measurable CFs remained rather constant. Neurons at a given constant collicular depth of more than about 400 microm showed a clear shift of CF from low to high, that is, they were tuned to the higher frequencies the older the animals were. Cochlear and collicular origins of this observed shift of tonotopy are discussed.