Noise exposure can affect the auditory system of a marine mammal by causing auditory fatigue, resulting in hearing loss (NIHL), which is an increase in the hearing threshold. If the loss is temporary, the NIHL is a temporary threshold shift. At extreme noise exposures, the hearing loss may remain creating a permanent threshold shift. The growth and onset of NIHL is a function of the duration, level, and frequency of the noise exposure, as well as the susceptibility of the species. Nonauditory physiological effects can also occur in animals in response to noise exposure. Hypothesized effects include nitrogen and fat emboli formation and the potential for the development of decompression sickness. Better-known effects include cardiac and endocrine responses (stress hormones). Interpreting cardiac responses to noise exposures is complicated due to dive-related bradycardia (reduction in heart rate) common to marine mammals, and the potential for noise habituation. Similarly, although cortisol is known to increase when marine mammals are stressed, little evidence exists that noise predictably elevates cortisol in marine mammals. Studies of cardiac and endocrine responses to noise are in their infancy and will likely yield more conclusive results as experimental procedures evolve.