Conditioned fear responses to a tone paired with footshock extinguish when the tone is presented repeatedly in the absence of shock. Rather than erase the tone-shock association, extinction is thought to involve new learning accompanied by inhibition of conditioned responding. Despite much interest in extinction from a clinical perspective, little is known about the neural circuits that are involved. Although the prefrontal cortex has a well established role in the inhibition of inappropriate behaviors, previous reports have disagreed as to the role of the ventromedial prefrontal cortex (vmPFC) in extinction. We have reexamined the effects of electrolytic vmPFC lesions made before training on the acquisition, extinction, and recovery of conditioned fear responses in a 2 d experiment. On Day 1 vmPFC lesions had no effect on acquisition or extinction of conditioned freezing and suppression of bar pressing. On Day 2 sham rats recovered only 27% of their acquired freezing, whereas vmPFC-lesioned rats recovered 86%, which was indistinguishable from a control group that never received extinction. The high recovery in lesioned rats could not be attributed to decreased motivation or altered sensitivity to footshock. vmPFC lesions that spared the caudal infralimbic (IL) nucleus had no effect. Thus, the vmPFC (particularly the IL nucleus) is not necessary for expression of extinction, but it is necessary for the recall of extinction learning after a long delay. These data suggest a role of the vmPFC in consolidation of extinction learning or the recall of contexts in which extinction took place.Key words: extinction; infralimbic; prelimbic; fear conditioning; amygdala; inhibition Prefrontal cortex has long been implicated in inhibition of inappropriate responses. Lesions of medial prefrontal cortex (mPFC) cause perseverative responding in animals and humans and cause deficits in reversal tasks (for review, see Kolb, 1984;Fuster, 1997). Perseverative responding in prefrontal animals was extended to conditioned fear when it was shown that rats with mPFC lesions could acquire freezing responses to a tone paired with a footshock, but they required many more days to extinguish those responses when the tone was presented alone (Morgan et al., 1993). More recent data support the hypothesis that mPFC is involved in the inhibition of fear responses (Bremner et al., 1999;Herry et al., 1999;Morrow et al., 1999), and it has been suggested that deficits in extinction of conditioned fear may cause certain anxiety disorders (Charney and Deutch, 1996;Pitman, 1997).Many questions remain, however, concerning the role of the ventromedial prefrontal cortex (vmPFC) in extinction of fear. Following the original report by Morgan et al. (1993), Gewirtz et al. (1997 found no effect of vmPFC lesions on extinction of conditioned fear responses. To explain this discrepancy, Gewirtz et al. (1997) suggested that the prolonged extinction might have been attributable to increased acquisition in the lesioned animals, which was masked by asymptotic freezi...

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