The present study was undertaken to examine how two ligands of the benzodiazepine receptor, which possess anxiolytic or anxiogenic actions, affect both the duration of immobility and the incidence of straw-climbing behavior in rats in a modified forced-swim test. Rats were injected IP with either vehicle, diazepam (0.5, 1, 5 mg/kg), or β-carboxylic acid ethyl ester (β-CCE; 0.5, 1, 2, 5 mg/kg), or a combination of diazepam at 1 mg/kg and β-CCE at 2 mg/kg. In addition, Ro 15-1788 (1 mg/kg), a specific benzodiazepine antagonist, was injected IP 20 min after diazepam injection and immediately after β-CCE injection, respectively. In the first 5-min period of the forced-swim test, diazepam at 5 mg/kg prolonged the duration of immobility, whereas β-CCE at 1, 2 and 5 mg/kg reduced its duration. Immediately after the first 5-min test period, 4 straws were suspended above the surface of the water, and the number of straw-climbing attempts and the duration of immobility were measured for a subsequent 5-min test period. Straw-suspension elicited straw-climbing behavior in forced swimming rats, resulting in a shortening of the duration of immobility in this period. All doses of diazepam inhibited straw-climbing attempts and prolonged the duration of immobility in a dose-dependent manner. β-CCE at 1 or 2 mg/kg enhanced straw-climbing attempts, but did not significantly affect the duration of immobility. Furthermore, the combined administration of diazepam and β-CCE antagonized the respective drug effects on the duration of immobility and the number of straw-climbing attempts. Moreover, the inhibitory effect of diazepam (1 mg/kg) and the enhancing effect of β-CCE (2 mg/kg) on straw-climbing attempts were reversed by Ro 15-1788 at 1 mg/kg, respectively. These results suggest that the two benzodiazepine ligands, which possess opposite effects on anxiety, could affect both the duration of immobility and straw-climbing attempts of rats in a modified forced-swim test via benzodiazepine receptors. It is further suggested that the immobility in this test may reflect coping behavior related to emotionality (i.e., anxiety and/or fear) and that the straw-climbing behavior may represent escape attempts from the water. Therefore these measures may be new indices of anxiety-related behavior.