From: USDA Animal Welfare Act Regulations, 9 CFR Part 3 (Standards), Section 3.8 (Exercise for dogs), paragraphs (a) and (b):
“(a) Dogs housed individually. Dogs over 12 weeks of age, except bitches with litters, housed, held, or maintained by any dealer, exhibitor, or research facility, including Federal research facilities, must be provided the opportunity for exercise regularly if they are kept individually in cages, pens, or runs that provide less than two times the required floor space for that dog, as indicated by Sec. 3.6(c)(1) of this subpart.
(b) Dogs housed in groups. Dogs over 12 weeks of age housed, held, or maintained in groups by any dealer, exhibitor, or research facility, including Federal research facilities, do not require additional opportunity for exercise regularly if they are maintained in cages, pens, or runs that provide in total at least 100 percent of the required space for each dog if maintained separately.”
See the source document in text or PDF format at the National Archives and Records website (home page is http://www.gpoaccess.gov/index.html). The document can also be accessed at www.iacuc.org.