Eighteen appropriate-for-gestational-age premature infants with birth weights ranging from 0.77 to 1.89 kg received 1 gm/kg of body weight of fat emulsion, intravenously, over a four-hour period. Infants less than 1 week of age developed a significant decrease in PO2 levels (P < 5.0) during the fat infusion period. There were no changes in other pulmonary function parameters. Infants less than 1 week of age also developed significantly higher peak levels of plasma triglycerides than infants 2 to 3 weeks old (P < .05). A correlation between increment in triglyceride levels and postnatal age was demonstrated (r = .75), with the younger infants presenting the higher triglyceride levels. This study demonstrates that: (1) small premature infants receiving intravenous fat are more susceptible to hyperlipemia and hypoxemia during the first week of life; (2) hypoxemia associated with intravenous fat infusion does not result from changes in lung dynamics; (3) the capacity to tolerate intravenous fats is enhanced after the first week of life.