The recent discovery of hair cell regeneration in the avian inner ear raises the possibility that hair cell regeneration might occur in the mammalian cochlea as well. The authors used 3H-thymidine labeling to detect mitotic activity in the cochleas of normal 3-week old gerbils exposed to acoustic trauma. Following an acoustic insult that caused progressively more severe damage in an apical to basal progression,3H-thymidine was injected for 5 days. Control animals were not exposed to the acoustic insult. The gerbils' cochleas were sectioned and processed for autoradiography. In the control cochleas, there were extremely rare labeled cells in the stria, the spiral ligament, and the glial cells around the acoustic nerve fibers. In the damaged cochleas, no evidence of hair cell regeneration or of any cell division within the normal sensory epithelial structures was seen. Three labeled cells were seen in intercellular spaces within the sensory epithelium; they appeared to be macrophages. Frequent cell division was seen in numerous other regions of the damaged cochleas and among glial cells adjacent to the acoustic nerve fibers. It is concluded that there is no evidence for hair cell regeneration following acoustic trauma in the gerbil, but acoustic trauma does induce cell division in numerous other areas of the cochlea.