Differences in drug pharmacokinetics between newborn and adult mammals are reviewed. The pharmacokinetic alterations during the maturation process are related to changes in the pattern of absorption, distribution, metabolism, and renal excretion. The most pronounced feature in neonates vs adults is the prolonged elimination half-life of drugs. The main factors causing delayed elimination are under-developed renal clearance and immature metabolism of drugs. Special attention has to be paid to central nervous system depressants and to drugs that are extensively metabolized because they will accumulate with repeated dosing of newborn animals.