1. Nine subjects recorded oral temperature and collected unstimulated submandibular saliva and submandibular saliva stimulated by sour lemon drops at a constant flow rate of 1 ml./min, at about 07.00, 11.00, 14.00, 17.00 and 22.00 h daily for time spans of about 11 days. 2. Least-squares cosine waves were fitted to the data to test for the presence and characteristics of circadian rhythms and the results were subjected to cosinor analysis and the Rayleigh test to assess the statistical significance of any rhythms for the group of subjects as a whole (i.e. whether or not the computed acrophases were randomly distributed throughout the 24 h of the day). 3. Unstimulated submandibular saliva showed circadian rhythms, significant for the group as a whole, in flow rate and in the concentrations of sodium, potassium, magnesium, chloride and inorganic phosphate but not in protein or calcium. 4. Stimulated submandibular saliva showed circadian rhythms, significant for the group as a whole, in sodium, potassium, calcium, magnesium, chloride and inorganic phosphate but not in protein. Oral temperature also showed a significant circadian rhythm. 5. Because of the high amplitude of many of these rhythms, they must be taken into account when establishing the normal range of salivary values, or when salivary composition is to be used as an indicator of systemic disease or to be implicated in the aetiology of oral disease.