Four experiments were conducted with chickens to examine the effects of chlordiazepoxide on tonic immobility, which has been implicated as an innate fear response. Not only did chlordiazepoxide produce a paradoxical dose-dependent increase in the duration of tonic immobility, but birds treated with chlordiazepoxide showed significantly enhanced shock-termination thresholds. Using two separate tolerance paradigms, the enhancement due to chlordiazepoxide was shown to be independent of the sedative and/or muscle relaxant effects of the drug. These findings have interesting implications for the supposed anxiolytic effects of the benzodiazepines and the relationship between fear and serotonin in avian species.