A number of chronobiological principles, as they pertain to the practice of laboratory medicine, have been discussed. Without doubt, salivary steroid assays are valuable tools for use in clinical studies, not necessarily as a single time-qualified sample but more often as a series of samples that will describe the underlying endocrinological time-structure. It is also important to appreciate that melatonin, certain drugs, and many dietary constituents such as phyto-oestrogens, are among the myriad of substances that can be monitored, and so hold great promise for the chronobiologist interested in the use of saliva as a sampling medium, both in the present and in the future.