We have examined the level of on-going cell death in the chick vestibular epithelia using the TUNEL method and compared this to the rate of on-going cell proliferation. Utricles contained 22.6 +/- 6.8 TUNEL-labeled cells (mean +/- s.e.m.) while saccules contained 15.1 +/- 4.0, with approximately 90% being labeled hair cells. In separate experiments, chicks were given a single injection of BrdU and killed 2 h later. Utricles contained 116.9 +/- 6.5 BrdU-labeled cells (mean +/- s.e.m.) and saccules contained 41.0 +/- 2.2. After 24 h in culture, utricles treated with 1 mM neomycin contained 115.5 +/- 38.9 TUNEL-labeled cells, an increase of 270% over controls. After 48 h, neomycin-treated saccules contained 40.9 +/- 7.8, an increase of 152% over controls. The majority of labeled cells were in the hair cell layer. Thus, neomycin exposure results in an apoptotic death of hair cells. The in vivo data measured here were used to estimate that the average life span of utricular hair cells in young chickens is approximately 20 days, in sharp contrast to the life spans assumed for hair cells in humans.