Three experiments investigated whether background noise modified the startle blink reflex and prepulse inhibition (PPI). In Experiment 1 background noise was about 28 (ambient), 40, and 60 dB; in Experiments 2 and 3 background noise was ambient and 60 dB. Prepulses were 70-dB tones (Experiments 1 and 3) or a tactile stimulus (Experiment 2). Startle-eliciting stimuli were 94-dB noise (Experiments 1 and 2) or an airpuff to the face (Experiment 3). Prepulses were presented at stimulus onset asynchronies of 30, 60, 120, 240, and 2000 ms relative to the startle-eliciting stimulus. Background noise of 60 dB had no effect on control reflexes, whereas 40-dB background noise increased control reflexes. Background noise decreased PPI, but only when an acoustic stimulus was used as prepulse. In sum, the findings show that background noise reduces the signal-to-noise ratio of acoustic prepulses and decreases their modulatory effect on the startle reflex.