The easy stress-free, non-invasive nature of saliva collection makes it one of the most accessible body fluids and it is potentially of value in studying normal human physiology as well as pathology. Measurements of salivary hormone levels will usually only be of value if they reflect the plasma level of the hormone and the relationship between the saliva and plasma levels of many hormones have been studied by a number of groups. The measurement of the salivary level is a valuable clinical tool for some hormones (e.g. cortisol, oestriol, progesterone), is of little value for others (e.g. cortisone, dehydroepiandrosterone sulphate, thyroxine, pituitary hormones) and for many others the saliva/plasma relationship is not yet sufficiently understood to assess the value of the salivary measurement. As well as reviewing the state of our knowledge of the salivary concentration of many hormones this review outlines a number of "rules of thumb" concerning the presence of hormones in saliva, their saliva/plasma relationship and the potential usefulness of assays of their salivary concentration.