The concentrations of amobarbital in saliva and serum were determined in 5 normal adults following ingestion of 120 mg of sodium amobarbital. There was excellent linear relationship between amobarbital concentrations in saliva and serum (r = 0.993); salivary levels were 36.1% of serum levels. Since the pH of saliva was generally lower than that of blood in man. the degree of ionization of amobarbital in serum and saliva had to be taken into consideration. Estimation of the protein binding of amobarbital in serum from concentrations of amobarbital in saliva and serum was in good agreement with the in vitro data of equilibrium dialysis.