After receiving milk-based formula for one week, 16 preterm infants, weighing 1,300 to 1,800 gm, were fed two isocaloric formulas containing either medium-chain or long-chain triglycerides for 15 days; the alternate formula was given for a second period of identical duration. While receiving MCT, the infants had greater (P smaller than 0.01) percent fat absorption (83.4 leads to 97.1%) and weight gain (7.5 leads to 11.5 gm/kg/100 calories). Because metabolic acidosis occurred with the LCT formula, the chloride content was adjusted to that of the MCT were confirmed and, in addition, there was a higher (P smaller than .01) percent retention of nitrogen (67.3 leads to 82.1).