We have previously reported in premature infants receiving intravenous fat infusions the changes in Lipo (a) that occur during intravenous feeding and during the initiation of enteral feeds (Pediatric Research, April 95) These infants were also investigated prospectively, to determine the changes in Apo B, Apo CIII, Apo A-1, and Apo E during the fat infusions and during the initiation of enteral feeds. 11 infants wereinvestigated (mean±SD gest age = 26±2 weeks, birth weight = 1019± 358 gms). Each infant received 1 gm/kg of 10% lipids over 18 hours on the first study day, followed by a daily dose of 2 gm/kg/day over 18 hours. Infants were begun on enteral feeds as decided by their caretakers. Daily samples were obtained pre and post infusion for triglycerides, and apoproteins A1, CIII, B, and E. All patients were studied for their entire hospital stay (duration of study = 68±26 days, range 25 - 99) In the period during Intralipid infusions prior to the initiation of enteral feedings, 5/11 infants demonstrated significant elevations in Apo B in relation to the expected rises in tryglyceride levels; the range of elevation was 2 - 3 times baseline values. Apo CIII rose in 10 of the 11 patients from 2 - 20 days after initiation of enteral feeds, in the range of 2 - 4 times baseline values. In contrast there was no elevation of Apo E during the study period, either with intravenous fat or enteral feedings. As expected Apo A-1significantly increased with the initiation of enteral feedings. These data suggest that intravenous fat infusions may stimulate either de novo synthesis or decreased catabolism of VLDL/LDL particles, because of the rise in Apo B levels. However, only with the initiation of enteral feeds were elevations in ApoCIII levels seen. Since this apoprotein inhibits tryglyceride breakdown and is atherogenic, this suggests an early risk factor for atherogenesis. Hepatic stimulation in the premature infant occurs immediately after initiation of enteral feeds. The liver then seems to acutely produce apoproteins CIII and Apo A-1 and Lp (a). (funded by Amer Ht Assoc)