This system of radio location has been used on rabbits (Sylvilagus floridanus), striped skunks (Mephitis mephitis), and raccoons (Procyon lotor). Size, weight, life, physical and electrical stability, and radiated power are important parameters of the transmitter. Mercury cells provide about 115 milli- watt hours per gram and were chosen as the transmitter power source. The transmitter is a transistor, crystal-controlled oscillator with the tank coil for the oscillator acting also as a magnetic dipole trans- mitting antenna. Antenna efficiencies of the order of 0.1 percent are obtained, yielding radiated power in the sub-microwatt region. The antenna is constructed of copper or aluminum and has a figure eight directional pattern. The complete transmitter without batteries weighs approximately 10 grams and costs approximately $8.00 for parts. Instructions for building transmitters are given. Transmitters were fastened to rabbits and skunks by means of harnesses and to rabbits, skunks, and raccoons by collars. The harnesses did not prove satisfactory for long-term attachment. The receiver is a portable, battery-powered unit weighing 10 lbs including the batteries. It is crystal controlled on 44 switched channels with a 4 db noise figure and a 30 CPS bandwidth. A re- ceiving system was assembled with commercially available units and used successfully for tracking skunks. The components of this system are a military surplus receiver (BC-453), a radio range filter (FL-13), and two transistorized radio frequency converters. The portable receiver is well adapted for tracking on foot using the null of its attached loop antenna for obtaining bearings on the animals carrying transmitters. The transmitters operate on different frequencies to avoid interference in taking bearings and to provide for individual identification.