The developmental gene expression of pancreatic lipase (PL) and its related proteins (PLRP1 and PLRP2) is anticoordinate. It is unknown whether dietary fat regulates the expression of these proteins in the preweanling stage. For determining the regulation of development and diet on PL, PLRP1, and PLRP2 as early as the suckling period, pregnant (Sprague-Dawley) rats consumed from day 15 (d15) of pregnancy through d9 of lactation a purified low (11% of energy) safflower oil diet [low-fat (LF)]. From d9 of lactation, dams and their respective pups were fed LF, mediumfat (MF; 40% of energy), or high-fat (HF; 67% of energy) safflower oil diets to d56. Milk fatty acid content had 15-to 100-fold less C:10 and 2.6-to 3.3-fold more C18:2 in MF and HF groups. Diet (LF Ͻ MF ϭ HF; P Ͻ 0.002), postnatal development (d15 Ͻ d21 Ͻ d28 ϭ d56; P Ͻ 0.001), and interaction of diet ϫ development significantly affected PL activity starting as early as d15. PL mRNA levels showed a parallel effect of diet (LF Ͻ HF ϭ MF; P Ͻ 0.013) and development (P Ͻ 0.001). Both PLRP1 and PLRP2 mRNA levels were significantly affected by development (P Ͻ 0.001) and had an anticoordinate pattern compared with PL expression (d15 Ͼ d21 Ͼ d28). Reported for the first time is the significant down-regulation of PLRP2 mRNA levels by high polyunsaturated fat in suckling (d15) Dietary fat provides the major energy (~50% calories) during infancy in breast milk and formulas (1). During the past decade, special attention has been given to the type of fat necessary for normal infant development, especially to longchain polyunsaturated fatty acids (PUFA). PUFA derived from essential fatty acids (C18:2, linoleic acid, and C18:3␣-linolenic) play a key role in brain and normal retinal development (2).Fat digestion and absorption in the infant and premature neonate depend on the developmental pattern of lipases (3).The digestive system continues to develop after birth in infants, and especially so in premature neonates, influencing their ability to digest fat. Gastric lipase, which starts the digestion of dietary fat and accounts for 10 -30% of fat digestion (4), pancreatic colipase-dependent lipase, and milk bile saltstimulated lipase all have potential roles in digestion of milk fat in the neonate (3). Pancreatic lipase-related protein 2 (PLRP2) may play a role in neonatal fat digestion as well (5). Although the pancreatic lipase levels in the newborn and even more so in the premature infant are low, the digestion of milk fat depends on these four lipases with unique and only partially overlapping functions (5-7).The exocrine pancreas synthesizes and secretes pancreatic lipase and two pancreatic lipase-related proteins; PLRP1 and PLRP2. PL and its related proteins have been identified in humans and rats. The PLRP1 is highly homologous to PL with conserved serine and histidine residues in the active site (8,9); however, this protein does not exhibit colipasedependent lipolytic activity when its full-length cDNA is expressed in COS or Sf9 cells (8,9). Although PLRP1 i...