Cardiac physiology is emphasized in many undergraduate physiology courses, but few nonmammalian vertebrate model systems exist that 1) can be studied fairly noninvasively, 2) are well suited for controlled experimentation, and 3) emphasize principles characteristic of the vertebrate heart. We have developed an inquiry-based undergraduate/graduate-level laboratory in cardiac physiology and electrocardiography using rainbow trout ("Oncorhynchus mykiss" Walbaum) and the BioPac MP30 data-acquisition system (other fish species and/or electrocardiographic recording devices can be substituted). This laboratory facilitates intensive study of vertebrate electrocardiograms (ECGs) under a variety of environmental and physiological perturbations and is ideal for use in multisession, inquiry-based laboratory projects in animal physiology. Furthermore, students gain valuable experience in scientific inquiry, study design, following and/or developing scientific protocols, and animal care. This laboratory requires the ability to keep captive fish of at least 100 g and equipment to record ECGs. Departments meeting these requirements can adopt this technique at modest expense. Student enthusiasm and feedback were positive, and several students commented that the nonlethal methods used added to the laboratory's perceived value. (Contains 3 figures and 4 tables.)