A federal judge last month ruled that animal-rights activists have the legal right to challenge U.S. Department of Agriculture rules that exempt the vast majority of research animals from federal regulation. Observers say that the ruling almost guarantees that the agency will extend regulations governing animal handling and housing to thousands of academic and industry laboratories that work with rodents and birds. Those new rules, say animal-care experts, could impose costly new requirements on labs that don't meet standards set by the private Association for Assessment and Accreditation of Laboratory Animal Care.