Application of microwave energy delivered through a probe and a catheter in open- and closed-chest dogs to induce atrio-ventricular block is described. Several microwave antennas/catheters with a preferential power deposition at the tip were designed, constructed and tested. These antennas were designed for operation at 2450 MHz. Measurements in saline and tissue-equivalent liquid phantoms showed greatly increased heating at the tips of these antennas. The antenna consisted of flexible coaxial cables (1.78 mm OD) with a thin, semigrid brass sleeve just proximal to the tip. The entire antenna was insulated except for the tip and could be used for recording bipolar electrograms. >