Measurements of specific activity in water from saliva and plasma samples taken simultaneously from the same person, following oral administration of tritiated water, were compared in a group of 34 adults, with a wide range of total body water values. The results showed that on average there was no detectable difference (P>0.10) between the specific activities of the plasma and saliva samples. Therefore, there is no significant difference between values for total body water derived from paired plasma and saliva samples. It is concluded therefore that saliva samples are a satisfactory alternative to plasma samples in the measurement of total body water using tritiated water.