A systematic quantitative set of data concerning the organ of Corti in developing Sprague-Dawley rats at intervals from 18 days of gestation to 10 days after birth (DAB) is provided in this study. Using phalloidin staining, the total number of inner and outer hair cells, the whole length of cochlea, as well as the diameter of inner and outer hair cells and the intercellular space between inner hair cells were determined in order to analyze the quantitative change of inner and outer hair cells during development and to explore some roles of the factors regulating the growth of cochlea. The results show that: (1) The length of cochlea approached its adult size by 7DAB. (2) The growth of the extreme part of the apex was responsible for the delayed elongation of the cochlea. (3) Growth in the cochlear length mainly results from an increase of cell diameter tempered by a decrease of intercellular space. (4) The adult size of inner and outer hair cells was obtained by 7-14DAB. (5) The final number of inner and outer hair cells was reached at 3DAB and remained constant through adulthood. No significant hair cell overproduction and cell death were observed during ontogenesis of the cochlea. The negligible importance of overproduction and missing hair cells during hair cell differentiation suggest that there is a precise regulation phenomenon for producing the right spatial organization of the organ of Corti.