Eighty rats received 10 light-shock pairings on two successive days. Seventy-two h after the final training session, subjects received lesions directed at the primary visual areas (deep and superficial layers of the superior colliculus, dorsal lateral geniculate nucleus, pretectal nuclei, visual cortex and thalamic reticular nucleus) and at the nuclei of the lateral lemniscus and reticularis pontis caudalis, proposed components of a primary acoustic startle circuit in the rat. Control animals were sham operated. One day later, all animals were tested for startle by presenting noise bursts of 3 different intensities in the presence or absence of the light conditioned stimulus. Potentiated startle (the difference between light-noise vs noise-alone trials) was significantly attenuated or eliminated by lesions directed at the dorsal nucleus of the lateral geniculate, deep layers of the superior colliculus, visual cortex, and the posteroventral region of the nucleus of the lateral lemniscus. Lesions directed at pretectal nuclei, superficial layers of the superior colliculus, thalamic reticular nucleus, nucleus reticularis pontis caudalis or dorsal nucleus of the lateral lemniscus did not attenuate potentiated startle. The results suggest that the visual pathway that mediates potentiated startle goes from the retina to the dorsal lateral geniculate nucleus to visual cortex to deep layers of superior colliculus and down to the postero-ventral region of the lateral lemniscus where acoustic startle is modulated.