The acoustic startle response (ASR) of male rats was measured during several sessions over a 24-hr period in both a light-dark cycle and a constant-dark condition. Each session consisted of 10 trials each at 80, 90, 100, 110, and 120 dB white noise. The results indicate robust daily and circadian modulation of ASR amplitude that consist of an approximately twofold nocturnal increase at eliciting-stimuli intensities above 80 dB. Similar results were observed in female rats in constant-dark conditions. To determine whether daily changes in auditory thresholds were responsible for the observed modulation, ASR reflex modification procedures were used. These procedures were designed to measure auditory thresholds at frequencies of 10 and 40 kHz at several times of day. The results suggest a lack of significant circadian differences in auditory thresholds at these frequencies. This study demonstrates a novel role of the rat circadian system in the modulation of ASR amplitude.