Triacylglycerols (TG) of rat milk supply about two-thirds of the energy consumed by suckling rat pups. The present studies were undertaken to determine stereospecific fatty acid composition and molecular species distribution of milk TG and TG produced during digestion, transported in lymph and blood, and present in the liver of 9-- 10-day-old pups. Results support non-random stereochemical fatty acid and molecular species distribution for all TG's analyzed. Stereospecific compositional results show loss of medium chain fatty acids (MCFA) during digestion, producing a shift to a larger average molecular weight TG than present in milk. These MCFA are esterified primarily at the sn-3 position of milk TG and appear to be hydrolyzed by the action of lingual lipase in the stomach. In vitro incubation of milk with tongue homogenate yields free fatty acids and glyceride products that resemble those found in suckling stomach contents. Further TG metabolism appears to involve redistribution of the long chain fatty acids that remain esterified in TG following gastric lipolysis and release of MCFA.