Brief electrical pulses applied to the rat tail elicit a complex vocal response which includes audible (peeps, chatters) and ultrasonic (USV) components. Aspirin and amitriptyline had no effect on the vocal responses. Morphine showed a dose-dependent and naloxone reversible antinociceptive effect on the 1st and 2nd audible peeps by decreasing their intensity (evaluated by their envelopes which correspond to the outer bounds of the soundwave amplitude plotted as a function of time), with ED50 values of 1.96 mg/kg and 0.36 mg/kg i.v. respectively. Paracetamol significantly reduced only the intensity of the second peep at the dose of 200 mg/kg iv. Chatter intensity was decreased by doses of 1 and 3 mg/kg i.v. of morphine which would suggest an effect on emotional components of pain. The intensity of USV was affected by morphine injection although the variations observed were non-significant. These data clearly implicate a specific role for the opioid analgesics in modifying the vocal pain related behaviors.