In 2 experiments, startle amplitude and startle stimulus-induced freezing (an index of fear) were measured in an acoustic startle response (ASR) paradigm in rats. Lesions to lateral tegmental tract (LTG), a pathway medial to brachium of the inferior colliculus (BIC), significantly decreased freezing and produced a persistent 5-fold increase in ASR amplitude compared with sham-operated controls. Lesions to BIC increased both ASR amplitude (2-fold) and freezing. Neither BIC nor LTG lesions affected startle amplitude when startle was elicited by a brief footshock stimulus. Characteristics of the lesion effects were tested with manipulations of interstimulus interval, stimulus intensity, and prepulse inhibition. The data suggest (a) an ascending pathway medial to BIC that carries the fear-inducing dimensions of an acoustic stimulus and (b) a descending pathway that provides tonic inhibition of the sensory input to the ASR circuitry.