We investigated the role of dopaminergic mechanisms in the attenuation of the acoustic startle response and prepulse inhibition (PPI) in rats by the introduction of unexpected changes in environment illumination. Experiment 1 showed that Dark-to-Light transitions robustly reduce startle responses and PPI. Experiment 2 showed that this phenomenon habituates across repeated testing sessions and reappears after an interval without testing. Experiment 3 demonstrated that haloperidol blocks the startle and PPI-reducing effect of the Dark-to-Light transition. We show how a computational model of acoustic startle response and prepulse inhibition can be extended to incorporate the empirical effects demonstrated in this study. We conclude that sensory gating as measured by prepulse inhibition is markedly attenuated in situations where novel stimuli are introduced during a test session and that dopaminergic systems may be involved in the dynamic changes evoked by the onset of illumination.