THERE ARE OBVIOUS ADVANTAGES to the physician if he is able to prescribe certain drugs in preparations which in one dose contain a sufficient amount of medication to last all day or all night. Although enteric-coated tablets probably represent the first attempt to alter the release of drugs, a variety of drugs are available now in prolonged, sustained, delayed, repeat-action, and timed-release dosage forms. Enthusiastic advertisements for some of these products, which may be generally considered as having prolonged-action, purport to describe just when and how much of a drug is released, and simplified curves of blood levels or clinical response claim to depict how the preparation will act in vivo. Since these products usually contain the equivalent of 3 normal doses of the drug, it is of considerable importance to the physician to know that the drug will actually be released in the advertised manner and not released either