This study is based on the hypothesis that the pharmacokinetics of antibiotics in neonates are different from other children and adults. We aim to study the population pharmacokinetics of neonates receiving the antibiotics for treatment of infectious diseases. In this study, we will detect drug concentration in plasma by using residual blood samples of blood gas analysis and employ computers for constructing population pharmacokinetic models. In addition, we also want to correlate use of antibiotics with treatment effectiveness and incidence of adverse effects in neonates. This novel knowledge will allow better and more rational approaches to the treatment of infectious diseases in newborns. It will also set the foundation for further studies to improve antibiotic therapies for neonates.