Bioavailability indicates the rate and extent of therapeutically active drug that reaches the systemic circulation and is available at the site of action. Bioavailability studies are carried out for both preclinical and clinical studies. Pharmacokinetic parameters generated from bioavailability studies including the rate and extent of drug absorption, distribution, metabolism and elimination are important for screening pharmacologically active intermediates and prodrugs, determining appropriate drug formulation and route of administration, and establishing recommended dosage regimens. In this dissertation, bioavailability studies were performed for S-carboxymethylcysteine and N-acetylcysteine, which are used in the treatment of chronic obstructive pulmonary diseases, and for (-)-carbodine, which shows high activity for many DNA and RNA viruses such as the Venezuelan equine encephalitis virus.