The relation between total noise-exposure energy, recovery time, or rest & % hair-cell (HC) loss was examined in 378 chinchillas. The continuous exposures were either a 4-kHz octave band of noise (OBN) at 47-108 dB SPL (N=169) for 0.5 h to 36 d, or a 0.5-kHz OBN at 65- 128 dB SPL for 3.5 h to 432 d (N=131). Recovery times varied from 0- 365 d. With both OBNs, other animals were exposed on interrupted schedules [6 h/d, /2d or /wk for 9-365 d (N=78)]. HC loss as a function of age in 117 non-exposed animals was used to correct for loss due to aging. For the 4- & 0.5-kHz OBN, the noise-exposed cochleas (N=607) were separated into 3 groups: 1) Acute (. 9 d exposure, 0 d recovery; N=90 & 56) to characterize the primary effects of the noise; 2) Chronic (> 9 d exposure, 0-730 d recovery; N=184 & 144) to determine secondary effects & 3) Interrupted (N=46 & 87) to show the effect of rest during the exposure. Cluster & regression analyses were performed in the basal & apical halves of the cochlea to determine the specific rates (relative to doubling of total energy, recovery & rest) at which primary & secondary effects produced HC loss & rest prevented loss. The effect of recovery time was isolated by subtracting the primary effect determined in the Acute group from the loss in the Chronic group. The effect of rest was isolated by subtracting the primary effect from the Acute group & the secondary effect determined from the Chronic group from the loss in the Interrupted group. It was found that: 1) When the OBN was above a critical level, there was no relation between total energy or recovery time & HC loss; 2) Below a critical level, there were highly significant log-linear relations, but at a low rate; 3) Except for the highest exposure levels, the majority of HC loss from the 4-kHz OBN occurred after the exposure had terminated, while that from the 0.5-kHz OBN occurred during the exposure & 4) Rest periods during either OBN exposure significantly reduced HC loss.