Zolpidem and diazepam are widely used drugs acting via benzodiazepine binding sites on GABAA receptors. While diazepam is non-selective, zolpidem has a high affinity for α1-, and no affinity for α5-containing receptors. Several studies suggested that behavioral effects of zolpidem might be more similar to classical benzodiazepines than previously thought. To compare the sedative and anticonvulsant properties of these drugs and to evaluate the importance of GABAA receptor subunits for development of tolerance during chronic treatment, we tested the effects of acute and repeated administration of zolpidem and diazepam on ambulatory locomotor activity (a measure of sedation) and on the threshold for myoclonic, clonic and tonic seizures in response to i.v. infusion of pentylenetetrazole (PTZ). Both drugs given acutely in doses 0.3, 1 and 3 mg/kg reduced locomotion, and in doses 1 and 3 mg/kg elevated the threshold for PTZ-induced seizures. The effects of zolpidem and diazepam on the tonic seizure threshold were greater than on myoclonus and clonic seizure threshold. Diazepam and zolpidem (3 mg/kg), given 18 or 42 h after repeated drug treatment (10 days, 5 mg/kg, twice daily), decreased the PTZ seizure threshold and increased the locomotor activity as compared to control mice, indicating development of tolerance to their anticonvulsant and sedative effects. After repeated treatment the PTZ seizure threshold was not different between the two drugs, while differences in sedation became larger than after the acute treatment. The results suggest that α5-containing GABAA receptors are not crucial for the development of sedative and anticonvulsant tolerance.