Effects of predictability of threat on potentiation of the startle reflex were investigated by presenting participants with predictable and unpredictable electric shocks. Shocks were presented either paired with a visual cue (paired condition) or unrelated to the presentation of the visual cues (unpaired condition). In contrast to previous slower-paced studies, conditions alternated at a rapid rate: each context lasted 8.5 s and within these contexts the visual cues had a duration of 1.5 s. Results replicated previous findings: in the predictable condition, startle responses were augmented by a threat-signaling stimulus, and startle responses in the unpredictable condition were larger than in a neutral condition in which no shocks were presented. In all three conditions, visual stimuli that did not carry information about when a shock could be presented augmented startle reactivity. A control experiment showed that the effects of threat on the startle response could not be ascribed to attention and that the effects of the lead stimuli that did not signal threat are likely to be unrelated to the effects of threat. These results show that the fear system is modulated dynamically as a function of rapidly changing information about threat and emphasize the role of predictability of an aversive stimulus in the distinction between cue-specific and contextual fear.