The new Guide for the Care and Use of Laboratory Animals recommends enrichment for rodents as a way to reduce the stress of confinement, crowding, and other stressors, by allowing the animal to exercise choice and control over its environment. In fact, enrichment should be provided across the entire animal program.
One important point is that enrichment programs must be reviewed by the IACUC, researchers and the veterinarian. The Guide implies that the enrichment program should be in writing, and that enrichment should be considered an independent variable which must be controlled within the research project.
Social housing is the default position of the guide and animals that must be singly housed, including rodents should be permitted access to larger enclosures with additional enrichment opportunities.
On a related note, personnel should be trained on animal behaviors and observation so as to appropriately monitor and report on abnormal behaviors. For additional information, see the FAQS at http://www.aaalac.org/accreditation/faq_landing.cfm#C
Take a look at our line of enrichment items for rodents.