Improved methods and devices perform tachycardia detection and anti-tachycardia pacing (ATP) to convert a tachycardia (e.g., VT or AT) to normal sinus rhythm. According to one embodiment, an anti-tachycardia pacing method includes sensing, during sinus rhythm, first and second cardiac signals at first and second sites, respectively, in a patient's heart. The first and second sites include left and right ventricles or left and right atria, for example. The method further includes sensing third and fourth cardiac signals at the first and second sites, respectively, during a tachycardia (e.g., ventricular tachycardia or atrial tachycardia). The cardiac signals are processed to provide respective values. One or more anti-tachycardia pacing pulses are delivered at the site closest to the reentrant circuit based on a comparison of a first ratio of the first and third values and a second ratio of the second and fourth values. Unipolar sensing of the cardiac signals may be employed by, for example, shorting together pairs of electrodes implanted at each site.