Since 1985, the US Animal Welfare Act and Public Health Service policy have required that researchers using nonhuman primates in biomedical and behavioral research develop a plan “for a physical environment adequate to promote the psychological well-being of primates.” In pursuing this charge, housing attributes such as social companionship, opportunities to express species-typical behavior, suitable space for expanded locomotor activity, and nonstressful relationships with laboratory personnel are dimensions that have dominated the discussion. Regulators were careful not to direct a specific (...) set of prescriptions (i.e., engineering standards) for the attainment of these goals, but to leave the design of the programs substantially up to “professional judgment” at the local level. Recently, however, the Institute of Medicine, in its path-finding 2011 report on the necessity of chimpanzee use in research, bypassed this flexible and contingent concept, and instead, required as a central precondition that chimpanzees be housed in “ethologically appropriate” environments. In so doing, obligations of ethical treatment of one great ape species were elevated above the needs of some research. The evolution and significance of this change are discussed. (shrink)