Animal-assisted therapy is growing in recognition in long-term aged care facilities because of the benefit it offers for alleviating negative behaviors and increasing socialization among residents. However, animalassisted therapy programs are often discouraged by environmental constraints. The purpose of this study is to create a design intervention to 1) facilitate human-animal interactions in a long-term aged care facility that endeavor to promote pet therapy, and 2) to determine the effects of environmental factors that may influence the beneficial outcome from human-animal interactions. A design intervention was created to support the spontaneous and pleasant nature of the pet visit therapy sessions. Sixteen residents participated in the study and were offered to interact, supported by the design intervention, six times with a chinchilla, three-day intervals. A total of 63 human-pet interactions were observed and analyzed. Participants' negative behaviors decreased significantly (NHBPS, p=.01). A consequence of the use of the design intervention was the identification of consequential environmental factors including time of the day and the level of privacy, both of which affected aged persons' interaction with the chinchilla. In the end, this study demonstrated that supportive environment design is of critical benefit in conducting successful animal-assisted therapies in long-term aged care facilities. Keywords: Animal-assisted Therapy, Pet-therapy, Design Intervention, Design for Aging, Healthcare Design