Primiparous pregnant Sprague-Dawley dams were administered a single daily s.c. injection of diazepam (0.1 and 1 mg/kg) or vehicle over gestation days 14-20. No differences in neonatal mortality and weight gain were found between the control and diazepam-exposed pups. Conversely, male pups prenatally treated with this benzodiazepine exhibited subtle behavioural alterations either during early postnatal life or during adulthood. In particular, a significant decrease in the locomotor activity of the diazepam-treated groups was found at the end of the second postnatal week (14-16 days). Furthermore, the administration of diazepam during gestation produced marked changes in the length of ultrasonic calls of rat pups removed from their nest. Finally, adult male rats (120 days of age) prenatally exposed to diazepam showed a notable impairment in copulatory activity as well as a significant decrease in the duration of ultrasonic (22 kHz) post-ejaculatory calls emitted during sexual behaviour. These findings suggest that late gestational exposure to diazepam induces both short- and long-term behavioural changes in rat offspring, changes characterized by altered activity patterns and emotional-motivational responsiveness to environmental challenges.