Comments & suggestions  should be directed to Visit my website Housing Cage space requirements for rats as listed in the most recent Guide are shown below. Body weight  (g) Floor area per animal (in2) Cage height (in)       <100 17 7      100-200 23 7      200-300 29 7      300-400 40 7      400-500 60 7       >500 70 7 Bedding material provides thermal insulation, absorbs fecal and urinary wastes, and in some instances is used for nest construction.  The material chosen should be absorbent, not readily eaten, free of infectious agents and injurious substances, and comfortable for the animals. Bedding may consist of paper, hardwood chips, or corncob materials.  The use of aromatic wood shavings such as pine and cedar shavings should be avoided in the laboratory setting as it induces activation of hepatic microsomal enzymes which may interfere with experimental results. Autoclaved bedding should be used for immunodeficient animals to prevent the introduction of opportunistic infectious agents.  The amount of bedding material provided is important.  Provision of too much material combined with digging and piling activities typical of rats can result in contact with the water source and a flooded cage. The use of microisolator tops on cages reduces the incidence of infectious disease transmission by air and also helps to minimize the formation of airborne allergenic particles. If microisolator tops are used, particular attention must be paid to cage cleanliness as these tops allow for build-up of excessive ammonia levels in dirty cages.  Ammonia is a respiratory irritant, and may interfere with some research studies as well as exacerbate diseases such as mycoplasmosis. The temperature in the rat room should range from 64°to 79°F (18° to 26°C) with an average temperature of 72F (22°C).   Rats that are singly housed may require slightly increased temperatures for comfort.  The relative humidity in rat rooms should be between 40% and 70%. Temperature and humidity conditions of the rat room are important as low humidity (less than 40%) and high temperature (greater than 80°F) can result in a condition known as ringtail.  Ringtail is characterized by the appearance of concentric rings around the tail and frequently results in sloughing of all or part of the tail.  The feet may also be swollen and reddened in affected rats.  Young rats are more readily affected than are adults. To ensure proper ventilation and removal of ammonia and odors, the minimum number of air changes per hour should be 10 - 15.  In some situations a greater number of air changes per hour may be required to maintain good air quality.  The constant presence of the odor of ammonia indicates that rooms are overcrowded or that a greater number of air changes are needed.  Keep in mind that the air in cages, especially those with microisolator tops, will be 1°to 4°F warmer,  5% to 10% more humid, and have a greater concentration of ammonia than the surrounding room air. Animal rooms should be regulated by automatic timers to provide cycles with 12 - 14 hours of light and 10 - 12 hours of dark.  Animals in breeding colonies should have 13 - 14 hours of light as this is thought to be optimal for the reproductive efficiency of rats.  Timers should be checked regularly, as constant light can interrupt estrous cycles.  Albino rats are more susceptible to retinopathy induced by excessive light levels than any other species.  Light levels of 30 foot-candles are recommended for albino animals to avoid retinal damage. Keep in mind that rats housed on the lower racks will receive less light than those housed on the top racks. Noise in the animal facility is a consideration for the management of rats, as loud noise may be stressful to rats.  Responses to constant loud noise may include reduced fertility, metabolic and cellular alterations, and behavioral changes due to cortisol release.  Rats should not be housed near loud equipment.