We used the bromodeoxyuridine technique to study the proliferative activity in the basilar papilla of normal and Belgian Waterslager canaries with and without preceding sound trauma. Without sound trauma, there were, on average, six supporting cell divisions per day in the basilar papilla of Waterslager canaries. This rate of supporting cell proliferation corresponds well with estimates of the rate of hair cell differentiation derived from counts of immature-appearing hair cells obtained by using scanning electron microscopy of the Waterslager basilar papilla. Thus, supporting cell division appeared correlated with hair cell differentiation in Waterslager canaries. Bromodeoxyuridine labeling of cells in undamaged non-Waterslager canaries also indicated a very low rate of supporting cell division. In contrast with Waterslager canaries, this low rate of proliferation was not associated with a measurable rate of hair cell differentiation. In both normal and Waterslager canaries, exposure to traumatizing sound induced a dramatic increase in the rate of cell proliferation. These data show that a very low rate of supporting cell proliferation is normally present in birds, but it is not associated with a corresponding rate differentiation of hair cells. Only an increase above this low ambient rate of supporting cell proliferation, such as that following loss of hair cells, induces the differentiation of new hair cells in birds. The reason why Waterslager canaries do not completely compensate for their inherited hair cell deficit of 30% is not clear, when they can clearly respond to additional cochlear trauma from noise exposure with an increase in proliferation rate.