THE BARBITURATES can be divided into two groups on the basis of their metabolism. One group is metabolized primarily by the liver. The short-acting drugs, in general, fit into this group. The second group comprises those barbiturates in which elimination of the active agent is primarily by renal excretion with inactivation by the liver being of less significance. Phenobarbital, allobarbital, aprobarbital, butallylonal, phenylmethybarbituric acid, and barbital are the agents eliminated primarily by the kidney. 1 In this country phenobarbital is the member of this group which is most commonly used clinically. Phenobarbital is excreted into the urine by means of glomerular filtration and tubular reabsorption without tubular secretion. Waddell and Butler 2 found that the excretion of phenobarbital is influenced by both urine pH and urine flow. On the basis of this finding they suggested that only the unionized, lipid-soluble form of the drug is reabsorbed, presumably by passive nonionic