The purpose of the study was to characterize the ontogeny of the acoustic startle response (ASR), and response sensitization to background noise, in preweanling rats. With constant low-level (45 dB) background noise, response latency decreased steadily with age, whereas, both response incidence and amplitude increased non-monotonically 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 to ones tested at 45 dB; the second compared the ASR of animals tested at 3 background levels (30 dB range) within the test session. In each instance, sensitization was first evident at 16 days of age. Results indicate that daily test experience does not alter ASR amplitude, latency, incidence or the development of sensitization.