A quantitative study of spiral ganglion neurones was performed in rats during postnatal days 4, 5, 6, 30 and 60. There are 25,194 +/- 462 ganglion cells on postnatal day 4, abruptly falling to 18,809 +/- 514 on the 6th postnatal day. This neuronal loss accounts for the 22% of the overall ganglion cell population. The number of neurones remains almost unchanged from the 6th to the 60th postnatal day. This numerical variation in the neuronal population of the spiral ganglion seems to be related to the changes that take place during cochlear synaptogenesis, at the end of the first postnatal week, on the base of the outer hair cells. These changes involve competition among efferent endings approaching the cell and some afferents connected with it at birth, that disappear as a result of such a competition.