In extinction of auditory fear conditioning, rats learn that a tone no longer predicts the occurrence of a footshock. Recent lesion and unit recording studies suggest that the medial prefrontal cortex (mPFC) plays an essential role in the inhibition of conditioned fear following extinction. mPFC has robust projections to the amygdala, a structure that is known to mediate the acquisition and expression of conditioned fear. Fear conditioning potentiates the tone responses of neurons in the basolateral amygdala (BLA), which excite neurons in the central nucleus (Ce) of the amygdala. In turn, the Ce projects to the brainstem and hypothalamic areas that mediate fear responses. The present study was undertaken to test the hypothesis that the mPFC inhibits conditioned fear via feedforward inhibition of Ce output neurons. Recording extracellularly from physiologically identified brainstem-projecting Ce neurons, we tested the effect of mPFC prestimulation on Ce responsiveness to synaptic input. In support of our hypothesis, mPFC prestimulation dramatically reduced the responsiveness of Ce output neurons to inputs from the insular cortex and BLA. Thus, our findings support the idea that mPFC gates impulse transmission from the BLA to Ce, perhaps through GABAergic intercalated cells, thereby gating the expression of conditioned fear.