Background Bile salt-stimulated lipase (BSSL), pancreatic triglyceride lipase (PTL), pancreatic lipase-related protein 2 (PLRP-2), phospholipase A2 and colipase are all expressed in the exocrine pancreas and involved in digestion of dietary fat in the small intestine. BSSL is also expressed in the mammary gland during lactation and is a constituent of the milk in some species, including human. The contribution of BSSL with the milk compensates the low intraluminal levels of PTL and bile salts in infants that otherwise may lead to fat malabsorption. In fact, such fat malabsorption is almost exclusively seen in formula-fed, but not in breast-fed infants. Aim To compare the ontogeny of the four pancreatic lipases and colipase in one species that secretes BSSL in milk (mouse) and one that does not (rat). Methods Quantitative real-time PCR and Western blots were performed to follow lipase expression in pancreas from birth until adulthood in both species. Results BSSL was expressed already at day 1 post-partum. In mice, the level of BSSL mRNA increased gradually from day 1 to reach a peak value around day 7, where after it declined. In contrast, the rat pancreatic BSSL appeared to be constitutively expressed already from day 1 until adulthood. Previous studies showing that PTL and PLA-2 expression is low until the suckling/weanling transition, whereas PLRP-2 and colipase is expressed from day 1 were confirmed in both species. Conclusion BSSL together with PLRP-2 are the dominating lipases involved in fat digestion as long as pups are suckled. Further, in mice lower expression of BSSL the first days seems to be compensated for by BSSL in milk.