The conditioned suppression procedure involves a Pavlovian pairing of a CS with an aversive US, such as an electrical shock. It has been used in a number of situations and is especially useful for studying the acqusition and extinction of conditioned fear. The researcher begins by training a rat to press a lever for food reinforcement. Once the rat is reliably lever pressing, the researcher presents a CS such as a tone or light, and terminates the CS with a brief footshock US. Fear is measured by the extent to which the tone elicits a freezing response and thus suppresses the rat's lever pressing. In the present procedure, the 15-s tone CS (designated with a musical note) is 80 dB. The shock US is mild (.4 mA) and brief (0.3 s). This video presents three tone-shock trials. The first two are the rat's first and second CS-US pairings; the third shows the rat freezing after 10 trials. (1:19)