This year our national expenditures for biomedical research and development will approximate$3.5 billion. It is estimated that about half of this effort will in some way involve theuse of animals. More than 45 million warm-blooded animals and millions more of lowerspecies are used annually in biomedical research. An estimated 16 million net sq ft ofbuilding space is used to house laboratory animals. This space must provide a variety ofenvironments which are suitable for the particular project in which the animal subject isbeing used. Animal facility ventilation is an important factor in the overall equation whichdefines the environment of an experimental animal. The program I am involved in at theNational Institutes of Health (NIH), namely, the Animal Resources Program of the Division ofResearch Resources, is broadly concerned with the problems of providing the best animal,correctly maintained in the appropriate environment for the extramural biomedical research programs of NIH. This is why we are concerned about animal facility environments and havesupported some of the experimental work reported at this symposium.