In this study, time-dependent change of whole human salivary viscosity and sialic acid content were investigated. Whole saliva samples were obtained from seven clinically healthy human males (aged 24 to 34 years) without any mouth cleaning for two days (48 hours). The saliva before and after 5min-brushing was collected between meals and immediately cooled down to below 0°C. The following tests were immediately for a part of the saliva and after 1, 2, 4, 6 hours of incubation period (37°C) for the rest of the saliva. The viscosity in Centipoises (c. p.) was measured at 37°C using model ELD cone-plate viscometer, with a shear rate of 76.80sec-1. Sialic acid was measured by the thiobarbituric acid assay using sialic acid as standard.The following results were obtained:1. Both salivary viscosity and sialic acid content showed a time-dependent decrease during incubation at 37°C. A more rapid decrease of both measures was observed in the saliva collected before brushing.2. There was no significant difference in the decrement of salivary viscosity between both groups within 2 hours. But a more rapid decrease was found in the saliva collected before brushing after 6 hours of incubation period.3. There were obtained statistically significant differences in the total and bound sialic acid content between the saliva before and after brushing at every time interval.4. A statistically significant difference was found in the free sialic acid content between the two groups only immediately after collection of saliva.5. A statistically significant positive correlation was found between the salivary viscosity and the bound sialic acid content only immediately after collection.6. Neither a decrease of salivary viscosity nor a degradation of bound sialic acid were obtained in sterilized whole saliva even after 6 hours incubation using the same assay condition.It was concluded that sialic acid degradation are closely related to the decrease in viscosity and presumably these phenomena are caused by salivary neuraminidase and aldolase activity.