In this chapter, the topic of medical chronobiology has been presented. Emphasis has been placed on issues related to clinical and occupational medicine. Results of initial studies conducted primarily by clinical pathologists indicate that the results of several commonly ordered diagnostic tests are influenced in a significant way according to the circadian time when samplings or tests are conducted. In addition, the pharmacokinetics and pharmacodynamics of medications can vary significantly according to their administration time. Indeed, the safety as well as efficacy of certain medications or dosage forms are circadian time dependent. A new thrust in the clinic is the application of chronotherapeutics to maximize the effectiveness and safety of medications. For the occupational health practitioner, the topic of medical chronobiology offers new perspectives to better address common problems experienced in clinical occupational medicine. First, the utilization of time-qualified (for circadian rhythmicity) reference values serves to differentiate with greater precision normal from unusual test findings; for example, among those arising during the pre-employment and scheduled follow-up physical examinations. Second, it provides better insight into the basis for the biological intolerance to rotating shiftwork. Too often, employee complaints of shiftwork intolerance are met with skepticism by members of the management and/or medical staff. The phenomenon of biological intolerance to rotating shiftwork is real. It has only been during the past decade that shiftwork intolerance has been demonstrated to have a chronobiologic basis. It seems that persons who possess a weak circadian time structure, that is, rather low-amplitude bioperiodicities, are more prone to develop biological intolerance to shiftwork later in life. Those endowed with a strong (high-amplitude) time structure are least prone. Thus, although persons with a weak circadian time structure might be at an advantage in tolerating rotating shiftwork schedules while young, they appear to be at a high risk of developing intolerance to it later in life. It is hypothesized that a weak circadian time structure predisposes one to desynchronization when subjected to work schedules entailing regular disruption of the activity-rest synchronizer schedule. The mechanisms underlying this biological intolerance to shiftwork require much additional investigation.(ABSTRACT TRUNCATED AT 400 WORDS)