Serum triglycerides, free fatty acids, unconjugated bilirubin, and albumin were evaluated in 40 neonates receiving 0.5-3.5 g/kg/day of a 50/50 soybean-safflower lipid emulsion infused during 18 h. The purpose of the study was to evaluate lipid tolerance and unconjugated hyperbilirubinemia according to our total parenteral nutrition protocol, which initiates lipid on postnatal day 4. Mean serum triglycerides and free fatty acids were within the range of prelipid infusion values at all dosages, and no statistically significant differences were noted between very-low-birth-weight neonates and those greater than 1,500 g birth weight. Mean free fatty acid:albumin molar ratio was less than 1.0 at all dosages and no individual patient values exceeded a ratio of 3.0. Mean peak serum unconjugated bilirubin of 5.8 mg/dl on postnatal day 3 was stable or fell the next 10 days of lipid-inclusive total parenteral nutrition. Initiating intravenous lipid on the 4th postnatal day at 0.5 g/kg/day and increasing at 0.5 g/kg/day increments at the end of the 1st postnatal week appears to be tolerated well. However, 5% of serum triglyceride levels exceeded 200 mg/dl. Therefore, in view of the unpredictability of a given patient's tolerance to lipid infusion, there should be monitoring for lipemia.