The pharmacodynamics in neonates are different from those in adults and children because the absorption, distribution, metabolism and excretion of drugs in neonates are always changing for the following reasons. In neonates, the proportion of extracellular fluid is large; the amount of plasma protein is small; renal function is immature; and the hepatic enzyme system is immature. In addition, individual birthweights, gestational ages in weeks, cardiopulmonary functions and renal excretory functions vary. All of these factors should be considered when selecting the dose and administration method of a drug. In concrete terms the distribution volume is large, which causes a low maximum concentration. In addition neonate renal excretory function is low and the hepatic enzyme system is immature, thus the half-life of drugs is prolonged. Therefore, the same dose per unit time as that for children (including infants) needs to be administered to neonates at dosing intervals that may be prolonged according to renal function.