The salivary half-life of antipyrine was used as a convenient procedure for estimating the relative rates of drug metabolism in man. The concentration ratio of antipyrine in plasma and saliva was one over a 24-hr period following the oral or parenteral administration of the drug to man and rat. Phenobarbital, a known stimulator of drug metabolism in animals and man, increased markedly the elimination of antipyrine from saliva of rats, while SKF-525A, a potent inhibitor of drug metabolism, prolonged the elimination of antipyrine from rat saliva. In addition, the known sex difference in the metabolism of drugs in the rat was detected by measuring the elimination rate of antipyrine from saliva of male and female rats. The clinical application of the procedure indicated that a group of epileptic patients treated with anticonvulsants for more than 2 mo had a mean antipyrine salivary half-life of 4 hr, whereas a mean half-life of 13 hr was found in a group of normal volunteers. The results show that the elimination rate of antipyrine from saliva is a useful index of drug metabolism in animals and man.