Special equipment was used to assess abrupt movements which characterize mammalian startle reactions to sudden acoustic inputs. Presence of background stimulation (random noise either at 50, 70, or 90 db. re .0002 dynes/cm2) facilitated startle to an intense tone (13,250 cps 117 db.). Other experiments indicated that startle to an intense pulse of noise (approximately 120 db.) is attenuated when a weaker pulse (approximately 80 db.) precedes primary stimulus by approximately 100 msec. Results indicate that startle is sensitive to sounds with intensities comparable to those encountered in a normal acoustic environment, and suggest that mechanisms involved in startle are more or less continually active even though overt reactions seldom occur.