Clinical standard 12-lead ECG recordings over 5 minutes on patients in atrial fibrillation or in atrial flutter were analyzed. After suppression of the signal components related to ventricular activity, the amplitude spectra of all leads were inspected. The spectrum of lead V1 clearly showed the presence of harmonics of a basic frequency, not necessarily at the modal frequency. The dominant basic frequencies in lead V1 of the patients in atrial fibrillation were (mean +/- SD): 604 +/- 1.1 Hz, range: [3.5 - 8.8] Hz. For lead V6 the values were: 4.79 +/- 1.13 Hz and [3.5-7.7] Hz. In several cases, the spectra suggested the presence of a secondary prominent rhythmic activity This could be verified by scanning the spectra of all leads for the presence of integer multiples of a different basic frequency. Among the standard 12-leads, leads VR, VL and V6 were the ones in which such multiple, prominent basic frequencies were most frequently identified.