Students' misconceptions about scientific phenomena can arise from at least two possible sources, the students' personal experience with those phenomena and things learned in the classroom. Misconceptions have been studied in a variety of science disciplines, but little attention has been given to the faulty models that students have for physiological processes. In this study 393 undergraduates in three different research universities were asked to predict the changes in heart rate and strength of cardiac contraction and breathing frequency and depth of breathing (physiological parameters that can be directly and personally perceived) under conditions that result in increased cardiac output or minute ventilation. One-third of those surveyed incorrectly predicted that heart rate would increase but that the strength of contraction would decrease or stay unchanged. Approximately one-half of the students predicted that breathing frequency would increase but depth of breathing would decrease (also erroneous). Explanations for these erroneous predictions were elicited, and the reasons offered revealed significant misconceptions about cardiac and respiratory mechanics. The persistence of such misconceptions was demonstrated in a small group of first-year medical students. A general approach to detecting and remediating misconceptions is discussed.