The purposes of this study were to quantify the insensible water loss that occurs across the Medtronic Minimax oxygenator and to estimate the resultant rise in fluid sodium concentration.A Carmeda-coated extracorporeal membrane oxygenation circuit connected to a Medtronic Minimax Plus oxygenator was primed with normal saline and attached to a closed reservoir. The gas sweep was randomly assigned to one of three rates: 2, 5, or 10 LPM (liters per minute). Each sweep rate was run in triplicate. The sodium concentration of the circuit was assessed after 12 and 24 hours of each trial. At the end of each 24-hour run, the evaporative loss was calculated. The average insensible water losses were 6.9+/-0.4 ml/h, 16.6+/-1.5 ml/h, and 34.4+/-0.3 ml/h at gas sweep rates of 2, 5, and 10 LPM, respectively (p<0.0001). Daily evaporative water losses for the membrane can be estimated to be 82.7+/-2.2 ml for each 1 LPM of sweep gas flow for a normal saline pump flow of 300 ml/min. In a closed circuit, a faster sweep gas rate is associated with a more rapid rise in sodium concentration (p<0.0001).