Four types of electrodes were studied with respect to efficiency of delivery of radiofrequency energy (RFE) and characteristics of the lesions produced in dog ventricular muscle in vitro. An imbeddable needle electrode was found to be the most efficient of the four types studied, with a lesion volume/energy-delivered ratio of 1.10 mm3/joule at an optimum power level of 2.8 watts and power density at the electrode surface of 0.45 watt/mm2. Lesion volume was linearly related to energy delivered at all power levels used below a power density of 0.64 watt/mm2, at and above which tissue impedance increased abruptly. The electrode catheter typically used for RFE ablations was the least efficient at 0.15 mm3/joule and a power density at the electrode surface of 0.50 watt/mm2 at 6.27 watts. masking 2/3 of the electrode surface with electrically insulating epoxy caused the RFE current to be directed into the tissue, resulting in a threefold increase in efficiency (0.46 mm3/joule) at less than half the power (2.79 watts). A flat, thermally insulated electrode with larger (10 mm2) contact area approached the efficiency of the imbedded electrode with a ratio of 0.69 mm3/joule at a low power density of 0.19 watt/mm2 and 1.85 watt power level. Characteristics of an "ideal" electrode for RFE lesion production in cardiac tissue are discussed.