The Homeostasis Concept Inventory (HCI) is a validated instrument for measuring students' knowledge of homeostasis. It is comprised of 20 multiple-choice questions covering key components of the previously validated Homeostasis Conceptual Framework (HCF). In this paper we present the first multi-institutional study of the impact of physiology instruction on students' HCI performance. Five cohorts of physiology or anatomy & physiology (A&P) students at four academic institutions took the HCI both at the start of their academic term (pre-test) and at the end of their term (post-test). Statistically significant but relatively modest improvements in overall scores were seen from pre-test to post-test. Among the 20 questions, eight questions had incorrect choices identified as "attractive distractors" on the pre-test, meaning that they were chosen at higher-than-random frequencies. From pre-test to post-test, there were only modest declines in selections of incorrect answers generally and of attractive distractors in particular. Three attractive distractors that all target one specific misconception -- that homeostatic mechanisms are active only when a regulated variable is not at its setpoint -- remained persistently attractive except for students of one instructor who directly addressed that misconception in lecture. These data are sobering in that they show a limited impact of instruction on HCI performance. However, these data also include encouraging evidence that instructional targeting of a specific misconception may help students overcome that misconception.