We evaluated the effects of extracorporeal membrane oxygenation (ECMO) on the pharmacokinetics of gentamicin in 18 infants who underwent ECMO therapy for severe respiratory failure and received gentamicin for possible sepsis. Twelve of these infants continued to receive gentamicin after ECMO had been discontinued. The volume of distribution (Vd) of gentamicin in the newborns receiving ECMO was 0.58 +/- 0.04 liter/kg, compared with a Vd of 0.45 +/- 0.02 liter/kg after ECMO had been discontinued (P = 0.02). The clearance of gentamicin in the patients undergoing ECMO was 42 +/- 3 ml/kg/hour compared with 57 +/- 4 ml/kg/hour in those patients off ECMO (P = 0.003). The elimination half-life in patients receiving ECMO was 10.0 +/- 0.7 hours compared with 5.7 +/- 0.4 hours after ECMO had been discontinued (P less than 0.0001). Neonates undergoing ECMO demonstrate a higher volume of distribution of gentamicin, a lower clearance, and consequently a longer half life for this drug. We conclude that gentamicin and probably other aminoglycosides should be given at dose rates about 25% lower than usual and at longer dosing intervals in patients undergoing ECMO therapy.