The hydrolysis of various dietary fats and pure triglycerides by pregastric esterase and pancreatic lipase was studied in vitro in conjunction with a series of digestibility trials to examine the effect of various chemical modifications to beef tallow on its digestibility in milk replacers for pre‐ruminant calves. Using pure triglycerides, it was shown that pregastric esterase preferentially hydrolyses triglycerides containing short chain fatty acids; the rate was highest with glyceryl tributyrate and decreased markedly with increase in chain length. Using butterfat, tallow and various modified forms of tallow, it was shown that the inclusion of butyric acid in tallow by interesterification, which markedly improved its digestibility, substantially increased its rate of hydrolysis by pregastric esterase. The initial rate of hydrolysis of tallow by pancreatic lipase was substantially less than that of butterfat. Little improvement in the rate of hydrolysis and solubilisation was obtained when the tallow sample was interesterified or when it was supplemented with the long chain fatty acids, myristic and oleic, or with the short chain fatty acid, butyric. A considerable increase in the initial rate of hydrolysis and solubilisation did occur with tallow samples with a reduced content of stearic acid. The results suggest that hydrolysis by pregastric esterase is a major site of discrimination between different fats in the pre‐ruminant calf and an important factor in determining overall digestibility.