Experiments were conducted in open-chest anesthetized dogs or in isolated arterially perfused canine atrial and ventricular preparations in vitro (see methods). Atrial fibrillation was induced in vivo using rapid pacing during stimulation of the right vagus nerve (2–4 mA at 10 Hz) and in vitro during perfusion with acetylcholine (1–3 µM). Ventricular fibrillation was induced in vitro using rapid pacing. Membrane potential was recorded in vitro by optical mapping using a voltage-sensitive dye (di-4-anepps). Standard 6.5F cardioversion catheters were used to deliver defibrillation shocks in vivo and in vitro. The shocks consisted of 1–5 symmetrical biphasic pulses of 8 ms duration at shock strengths of 20–100 V delivered via a custom-built cardioverter/defibrillator. Immediately following the optical mapping experiments, tissues were injected with 1–2 mL of Microfil contrast agent at 0.05–0.15 mL/min via the same cannula used for perfusion. The chambers were then filled with silicone to preserve tissue morphology during scans performed using a GE 120 micro-CT scanner with 25 µm x-y-z resolution to determine blood vessel sizes and distributions.