Acoustic startle responses (ASR) were studied in 12 young Wistar albino and in 15 hooded rats of both genders. The six week old animals were first exposed to a 6.9 kHz tone pair of 2 ms pulses of 120 db intensity with the inter-stimulus interval (ISI) between 2 and 11 ms. ASR amplitudes and latencies as a function of the ISI, animal strain and gender were recorded and analyzed for ten consecutive weeks. No differences in the ASR amplitude between Wistar and hooded rats were found. ASR amplitude increased during the experimental period and followed body weight increase. Significant differences were also observed between male and female rats in their startle responses to acoustic stimuli. Generally, male subjects responded with a greater ASR amplitude than females, and the changes may be attributed to the difference in neuromuscular development between genders. This experiment sets a background for further developmental studies.