Potential covariates were assessed for oral clearance (CL/F), oral volume of distribution in the peripheral compartment (V2/F), intercompartmental oral clearance (Q/F), and the first order absorption rate constant (ka). A two compartment model with parallel zero- and first-order absorption was found to best fit the data. A population pharrnacokinetic analysis was conducted for azithromycin on data from pediatric patients enrolled in four separate clinical trials. The second study is an example of the ability of the population approach to take advantage of sparse data obtained as a secondary objective of a clinical study. The first study was conducted on traditional pharrnacokinetic data (intense sampling) pooled from four pediatric trials. Two different examples of this approach are presented. Other than conducting traditional pharrnacokinetic clinical trials, another way to collect this information would be to perform a population pharrnacokinetic analysis. This new legislation required pharmaceutical companies to collect pediatric data for drugs with indications applicable to children before the compound would be approved. In the fall of 1997, the Food and Drug Administration proposed new legislation, commonly known as the ''Pediatric Rule". Specifically, population analyses are a way to gather information that might be difficult to capture in some subpopulations. The population approach to pharmacokinetic analysis, and its application to the identification of patient characteristics that affect a drug's pharrnacokinetic parameters, is achieving greater prominence in the drug development process.