Rhythmicity is a fundamental property of living matter: biological rhythms in man as well as in other organisms can be demonstrated at all levels of organization (population, group, individual, organ, tissue, cell and subcellular structure). From a macroscopic point of view a rhythm implies regular periodicity of data displayed as a function of time. With the help of computers and special programs, it is possible to obtain a microscopic characterization and quantification of rhythm. A biorhythm can be characterized by the statistical estimation of several parameters: the period, the acrophase, the amplitude, the mesor. Each of these parameters is expressed as a mean with its confidence limits for a degree of security of 95%. With reference to the period, rhythms can be analyzed as a spectrum with several spectral domains: circadian rhythms (with period of about 24 h) are the most carefully studied, but other rhythms exist such as ultradian rhythms (period 28 h, e.g. circamensual, circannual...). The acrophase is the peak time of the sine function used to approximate the rhythm. The amplitude is equal to half of the total rhythmic variability. The mesor is the mean adjusted rhythm; it corresponds to the 24-hour mean when sampling is performed at equal intervals. Circadian rhythms in blood variables are now fairly well documented in young healthy adults. Reports dealing with circadian rhythms in elderly subjects are few for obvious reasons, including the difficulty of gathering a group of elderly subjects comparable in age and social synchronization and moreover, free of major liver, heart, kidney, bone or any other degenerative disease and, consequently, not requiring medication. Therefore, we have undertaken a comparative study of the circadian rhythms in a group of 7 young healthy medical students (mean age 24 years) and in 18 elderly persons, free of the aforementioned diseases (mean age 80 years; 6 elderly men, 6 elderly women, 6 elderly women and men suffering from senile dementia). All subjects, either young or aged, were synchronized with respect to meal timing, diurnal activity and nocturnal rest. Blood samples were drawn at fixed times, every 4 h, during a single continuous span of 24 h beginning at 07.45 h (circadian rhythm) and the sampling was renewed four times a year to gather data on the circannual variations. Several biochemical variables were assayed, including electrolytes, enzymes, hormones. From these studies, it appears that the parameters characteristic of the biological rhythm may be modified with aging. However, the modifications are different from one function or variable to another. Differences in acrophases, mesors and amplitudes could be demonstrated. For instance, the acrophase may be shifted in the elderly for magnesium, calcium, proteins or some steroids, or on the contrary, it may be remarkably steady, e.g. for adrenal steroids, melatonin. The administration as a function of time of drugs or agents used as a diagnostic tool allowed to ascertain the importance of the time structure in this field. Both validity and interpretation of such administration depend upon the timing of the agent administration in the 24-hour scale. Therefore, the temporal organization of the subjects undergoing such treatments should be taken into account. One may presume that in a near future this type of investigation on various biological variables may lead to an improvement of the desired drug effects. Thus, the important changes observed in total serum proteins in the young and the aged, as well as the differences in the localization of their peak troughs, depending on the group, must be taken into consideration with regard to the roles played by serum proteins, in particular in the binding of several molecules, including hormones and drugs.