Noise in animal housing facilities is an environmental variable that can affect hearing, behavior and physiology in mice. The authors measured sound levels in two rodent housing rooms (room 1 and room 2) during several 24-h periods. Room 1, which was subject to heavy personnel traffic, contained ventilated racks and static cages that housed large numbers of mice. Room 2 was accessed by only a few staff members, contained static cages only and housed fewer mice. In both rooms, background sound levels were usually about 80 dB, and transient noises caused sound levels to temporarily rise 30-40 dB above the baseline level; such peaks occurred frequently during work hours (8:30 AM to 4:30 PM) and infrequently during non-work hours. Noise peaks during work hours in room 1 occurred about two times as often as in room 2 (P = 0.01). Use of changing stations located in the rooms caused background noise to increase by about 10 dB. Loud noise and noise variability were attributed mainly to personnel activity. Attempts to reduce noise should concentrate on controlling sounds produced by in-room activities and experimenter traffic; this may reduce the variability of research outcomes and improve animal welfare.