EXCITABLE media can support spiral waves rotating around an organizing centre1-7. Spiral waves have been discovered in different types of autocatalytic chemical reactions8,9 and in biological systems10-12. The so-called 're-entrant excitation' of myocardial cells13, causing the most dangerous cardiac arrhythmias, including ventricular tachycardia and fibrillation, could be the result of spiral waves1,2. Here we use a potentiometric dye14,15 in combination with CCD (charge-coupled device) imaging technology16,17 to demonstrate spiral waves in the heart muscle. The spirals were elongated and the rotation period, Ts, was about 180 ms (3-5 times faster than normal heart rate). In most episodes, the spiral was anchored to small arteries or bands of connective tissue, and gave rise to stationary rotations. In some cases, the core drifted away from its site of origin and dissipated at a tissue border. Drift was associated with a Doppler shift in the local excitation period, T with T ahead of the core being about 20% shorter thanT behind the core.