Thanks to an elaborated mathematical approach, based on statistics and signal processing, the chronological changes of the Erythrocyte Sedimentation Rate (ESR) of young healthy subjects, considered from a collective point of view, have been discriminated into genuine and well-defined rhythms. These rhythms are tied up either to natural (year, season) or 'social' (month, week, holidays) cycles, or to some other causes, still unknown and possibly intrinsic (such is probably the case of a 26.5-day strongly marked period). The solar induced time variation strictly obeys a frequency modulation law, the relative amplitude of which is 10 per cent. Two axes of symmetry are found, centered on 8 August and 8 February. The rhythm is roughly in accordance with seasons. The modulation frequency is maximum at summer time. Oscillations of the ESR are observed during the week and the month. Fridays and the last fortnight of each month appear to be low ESR time.