Infant and young adult squirrel monkeys (Saimiri sciureus) will have auditory and vestibular functions assessed before and after chronic lead and mercury intoxication. Hearing will be measured behaviorally by conditioned avoidance response techniques and electrophysiologically by computer-averaged nerve and brain evoked potentials (AER audiometry). Monaural air- and bone-conduction detection thresholds will be recorded from 125 Hz through 16,000 Hz. Vestibular function will be quantified by electronystagmography using bithermal caloric and body rotation stimulation. Positional and induced nystagmus will be measured in terms of duration, total number of eye beats and beat frequency. Following autopsy, temporal bone serial sections will be prepared and examined by light microscopy.