The purpose of our study was to determine whether a single administration of anticonvulsant doses of two ligands of benzodiazepine receptors, clonazepam and Ro 19-8022, leads to development of rebound phenomena in immature 12-day-old rats. Three tests were used: pentylenetetrazole (PTZ)-induced seizures, isolation-induced ultrasonic vocalizations, and motor performance. Susceptibility to the convulsant effects of PTZ decreased 24hours, but increased 48hours, after clonazepam administration. Ultrasonic vocalizations were completely suppressed 30minutes and 3hours after clonazepam; a moderate inhibitory effect persisted even at 48hours. Motor abilities were slightly compromised up to 3hours. Similar effects of Ro 19-8022 on PTZ-induced seizures and ultrasonic vocalizations were observed 24 and 48hours after administration; motor performance was not affected. Rebound proconvulsant effects followed different time courses after administration of the two benzodiazepine receptor ligands in developing animals. Anxiolytic-like effects of these drugs were still present at the time when animals exhibited rebound proconvulsant effects.