The effect of ventral medial frontal cortex (MFC) lesions on respiratory rate (RESP), immobility ("freezing"), and ultrasonic vocalizations (USVs) during conditioned emotional responses (CERs) was investigated. Male Sprague-Dawley rats were divided into two groups: MFC-lesioned rats (N = 11) sustained bilateral lesions of the infralimbic region of the MFC via microinjection of the neurotoxin NMDA; controls received sterile saline. Following a 2 week recovery period, all animals were differentially conditioned to two tones; a 2 mA footshock served as the unconditioned stimulus (US). The CS+tone was consistently paired with the US, while the CSr tone was randomly paired with the US. On the following day, RESP, freezing, and USVs were recorded during CER testing (no US were presented). All animals responded during the CS+ with increased RESP compared to baseline; the increase in MFC rats was significantly larger than in controls. All animals also froze at the onset of the CS+. Following the CS+, controls displayed a prolonged period of freezing (265 +/- 37 sec) and decreased RESP compared to baseline, and 92% emitted USVs. In contrast, MFC rats displayed a significantly shorter period of freezing (86 +/- 25 sec) and little or no USV; RESP remained significantly elevated throughout the remainder of the trial. These behaviors indicate a significantly altered stress response following ventral MFC lesions, implying that the MFC may be necessary for complete expression of various behavioral responses to "stressful" stimuli. The role of the MFC in emotion is also discussed.