The method presented in this paper allows the long-term recording of HR in an unrestrained animal, housed in the actual experimental cage, with permanently attached recording leads. Using this technique, it is possible to employ automated apparatus and programming equipment and to experiment with more than one animal at a time in a research area which has not been characterized by such procedures. Techniques for the construction of rat-proof leads, a terminal to which the leads are attached, a commutator, and the surgical procedure for implanting the terminal, were described.