The influence of a range of drugs acting at the benzodiazepine - GABA receptor - chloride ion complex on the ultrasonic calling of mouse pups was examined. Five benzodiazepine agonists decreased, and four inverse agonists increased, the rate of calling; both effects were blocked by the benzodiazepine antagonist RO 15-1788. Muscimol and baclofen, that act via the GABA receptor, both decreased calling, whereas pentylenetetrazole that directly affects the chloride channel increased the rate of calling. It was concluded that the monitoring of ultrasonic calling is a useful means of screening for the effects of benzodiazepines.