The cochleas of the chinchillas exposed to the four noise conditions described in the previous abstract were examined using the surface preparation technique. The animals (six) exposed to the continuous noise alone, had essentially normal hair cell populations. The animals (five) exposed to the 158‐dB peak impulse had hair cell losses that ranged from a scattered low level loss to approximately a 4 mm loss of OHC in the base of the cochlea. IHC losses were considerably smaller. The animals (six) exposed to the interrupted combination had nearly the same range of hair cell losses as seen in the 158‐dB group, with the exception of one animal that had a large (8 mm) discrete loss of OHC and IHC. The superimposed combination produced the most severe hair cell lesions with five of the six animals showing nearly 100% losses of OHC and IHC extending throughout the basal one‐third to one‐half of the cochlea. The audiometric and histological findings agree in showing that the superimposed combination of the two “safe” noises produces traumatic effects that more than exceed the additive effect of either component. The existing DRC do not provide guidlines for such combinations.