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The purpose of this study was to characterize the ontogeny of the acoustic startle response (ASR) and response sensitization to background noise in preweanling rats. Animals were tested daily from 11 to 21 days of age using one of four sets of background white noise levels [45–80 dB(A)]. With constant low-level (45 dB, SPL) background noise, response latency decreased steadily with age, whereas both response incidence and amplitude increased nonmonotonically with age. Two approaches were used to examine the ontogeny of sensitization to background noise: The first compared the ASR of animals tested at 75 dB background noise with ones tested at 45 dB; the second compared the ASR of animals tested at three background levels (30 dB range) within the test session. Sensitization was not evident before 15–16 days of age. By comparing these results with the results from naive animals, it was found that daily test experience does not alter ASR amplitude, latency, incidence, or the development of sensitization.