In a series of 6 experiments with a total of 160 male Sprague-Dawley albino rats, it was found that the startle response showed a progressive increase in amplitude when tones were presented against a high level of background noise. This sensitization effect was not a result of repetitive exposure to tones but rather a result of continuous exposure to noise. The size of the effect was directly related to noise intensity and required about 30-45 min to reach a maximum. The effect did not dissipate when the noise was maintained but did dissipate once the noise was turned down. Results are discussed in terms of the experimental conditions under which repetitive stimulus exposure produces either sensitization or habituation of the startle reflex. (PsycINFO Database Record (c) 2006 APA, all rights reserved)