One-pulse stimulation of the ventral amygdalofugal pathway (VAF), ventrolateral midbrain, or medial medulla evoked startle-like responses. Refractory periods tested by 2-pulse stimulation ranged from 0.2 to 0.5 ms in both midbrain and medulla sites and from 0.4 to 0.8 ms in VAF sites. Symmetric collision effects between midbrain and medulla sites suggest that fast-conducting axons between the sites mediate the response. Asymmetric collision effects between VAF and midbrain suggest that strong synapses between these sites mediate the response, with a transmission time of 1 ms. No collision was observed between contralateral sites. Bilateral lesions of midbrain sites blocked VAF responses and fear-conditioned startle, but failed to block acoustic startle, and partially blocked medulla responses. A new neural model of fear-potentiated startle is proposed.