The kidney is a major organ for drug elimination. The function of the neonatal kidney is markedly immature with a reduction of renal blood flow, of glomerular filtration and of active tubular secretion, even in healthy, term infants. Maturation of renal function in particular of glomerular filtration rate is gestational age and postnatal age-dependant. Moreover, many neonatal pathological conditions such as preterm birth, sepsis or perinatal asphyxia can also affect renal function. These developmental changes have a major impact on drug metabolism and elimination. Alterations in renal clearance can influence significantly both drugs efficacy and toxicity. Moreover, nephrogenesis is a still ongoing process in a number of premature infants before 36 wks postconceptional age. Drugs and toxic factors that may alter the constitution of the congenital nephron number endowment during this period may have long term consequences on arterial pressure and renal function at adulthood.