This study investigated whether exposure to cocaine during two periods of postnatal development affects the acoustic startle response (ASR) in adulthood. Female rats received 50 mg/kg/day cocaine HCl or vehicle SC during either postnatal days 1-10 or 11-20. At 60-65 days of age, subjects were ASR tested for 30 min on 2 consecutive days. Overall, ASR was increased on day 1 compared to day 2. Also comparisons between groups within each session and injection schedule showed that subjects exposed to cocaine during postnatal days 1-10 exhibited increased ASR amplitude on the second day of testing compared to controls. No group differences in response latency were observed. Therefore, these data indicate that, for female rats, cocaine alters the development of the pathways involved in modification of the acoustic startle response in a way consistent with a disruption of long-term habituation, but that the critical period for this disruption in females is postnatal days 1-10 and not 11-20.