Salivary sampling regimens are non-invasive, and therefore facilitate dynamic tests of hormone function and assessment of biological rhythms. Concentrations of neutral steroids in saliva are independent of flow rate and appear to reflect the non-protein-bound, 'free' fraction. Comparison of replicate determinations of quality control pools with determinations of samples collected at 2 min intervals allows the significance of short-term fluctuations in cortisol and testosterone secretory activity to be estimated. Samples collected at 15 min intervals provide a convenient way to estimate circadian rhythms, particularly in young children. Determination of salivary progesterone concentrations in samples collected by women daily, over extended periods of time, provides a valuable means of assessing ovarian function. Such assays may be used to monitor ovulation-induction therapy.