The purpose of this study is to use concept maps as an "expressed model" to investigate students' mental models regarding the homeostasis of blood sugar. The difficulties in learning the concept of homeostasis and in probing mental models have been revealed in many studies. Homeostasis of blood sugar is one of the themes in junior high school biology in Taiwan. However, few studies have investigated it. Here, semi-structured interviews were conducted together with concept mapping to explore students' mental models. There were 14 students, 7th- and 9th-graders, in this study. From the results, both groups were shown to lack a concept of the nervous system. However, 7th-graders did have more developed concepts of the endocrine and digestive systems. The data also indicated that students used external behaviours or life experiences to explain the mechanism responsible for the homeostasis of blood sugar, but did not possess a detailed view of the components of the mechanism. The implications, and recommendations for pedagogy, are discussed. (Contains 5 figures and 1 table.)