A recent positron emission tomography (PET) study on brain mechanisms in classical, or Pavlovian, conditioning is reviewed. The PET data were compared with skin conductance data recorded off-line. The participants were five men who participated in three different experimental phases. In the first (habituation) phase, a tone was repeated 24 times at random intervals. In the second (acquisition) phase, the tone was paired with a brief shock to the wrist. In the third (extinction) phase, the tone was presented alone again. Statistical parametric mapping analysis of the PET data showed significantly increased activation of the right hemisphere in the orbitofrontal cortex, dorsolateral prefrontal cortex, inferior and superior frontal cortices, and inferior and middle temporal cortices. In the left hemisphere, only Area 19 and the superior frontal cortex showed significant activation. The findings are discussed within a theoretical framework that argues for different brain mechanisms in acquisition and extinction in classical conditioning.