The circadian rhythms of twenty-one chemical serum parameters (albumin, alkaline phosphatase, calcium, carbon dioxide content, chloride, cholesterol, creatine phosphokinase (CPK), creatinine, glucose, glutamic-oxalacetic transaminase (GOT), gamma glutamyl transferase (Gamma--GT), lactate dehydrogenase (LDH), inorganic phosphorus, iron, potassium, total bilirubin, total protein, sodium, triglycerides, urea nitrogen, uric acid) and of urinary volume and oral temperature were studied, in October 1981, in a group of 49 elderly subjects (23 men, 73 +/- 6 years of age, and 26 women, 77 +/- 8 years of age) institutionalized at the Berceni Hospital for the aged. Statistically significant circadian rhythms as a group phenomenon were found in all functions except alkaline phosphatase, GOT, and LDH. The timing and the extent of these rhythms are presented. The circadian time structure of body chemistry appears well maintained in old age. Some circadian rhythms show a large enough amplitude to require the establishment of time qualified reference ("normal") ranges (e.g. serum iron). In most others, the circadian amplitudes are small and at present of little or no diagnostic importance. They are, however, of physiologic and pathophysiologic interest indicating an intricate time sequence of metabolic events in the human body.