Salivary, unbound and total plasma levels of diazepam have been compared in nineteen subjects. 2. Salivary diazepam levels were significantly higher (P < 0.001) than, but closely related to the corresponding unbound levels (r = 0.97) at 2.5-3 h after administration of the drug. 3. The results presented suggest that saliva levels can be used to predict the corresponding plasma levels at a given time, but there is no 1:1 relation between the unbound diazepam concentration in plasma and the saliva level of the drug.