The lateral, basal and central nuclei of the amygdala are important for conditioned fear, but not unconditioned fear to a number of predator odors (e.g., cat fur odor, trimethylthiazoline, TMT). This suggests that the neural circuits for unconditioned fear to predator odors maybe different from that involved in conditioned fear. An alternative neural circuit for unconditioned fear, the medial hypothalamic defensive circuit (MHDC), consisting of the anterior hypothalamic (AHN), ventromedial (VMH) and dorsal premammillary nuclei (PMd) has been proposed. Electrolytic and ibotenic acid lesions of the PMd have been shown to reduce unconditioned fear in rats presented with either a cat or cat odor. The present study compared the effects of electrolytic and neurotoxic lesions of MHDC nuclei in rats on unconditioned fear to TMT, as measured by freezing. Electrolytic lesions of the PMd did not reduce freezing in rats presented with TMT, but did have an overall effect on conditioned fear. Electrolytic lesions of the AHN and VMH, however, reduced freezing to TMT while not affecting conditioned fear. The results of the electrolytic lesion studies indicated a possible role for the AHN and VMH, but not the PMd, in unconditioned fear to TMT. Neurotoxic lesions of these two areas were then made to further examine the role of the VMH and AHN in TMT-induced fear. Neither NMDA lesions of the AHN nor ibotenic acid lesions of the VMH reduced freezing in foot shock-induced conditioned or TMT-induced unconditioned fear paradigms. The effect of electrolytic, but not neurotoxic lesion, of the VMH or AHN on TMT-induced unconditioned fear suggests that the effect was generated by the destruction of local axons or of fibers of passage from areas outside of the MHDC. These effects suggest that the MHDC does not play a role in TMT-induced fear.