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We have measured lipolytic activity in gastric aspirates obtained at birth in a group of 142 infants. The infants ranged in gestational age from 26 to 41 wk. Lipolytic activity, measured by the hydrolysis of long chain triglyceride ([tri-(3)H]oleate), and expressed as nanomoles FFA per milliliter gastric aspirate per minute was 333+/-66 in 55 small premature infants (gestational age 26-34 wk and body wt 750-2,000 g) and 558+/-45 in a group of 87 larger infants (gestational age 35-41 wk and body wt 2,020-4,000 g). No activity was detected in seven infants with an unusually low pH in the gastric aspirate, 2.88+/-0.44 (compared with a mean pH level of 5.59+/-0.22 in the other 135 infants). Attempts to characterize this lipase showed that it has a molecular weight of 44-48,000, pH optimum of 3.0-5.0, that FFA acceptors (albumin) stimulate activity, whereas bile salts, taurocholate and glycocholate, cause marked inhibition at concentration >3 mM. Our survey shows that enzyme activity is present as early as 26 wk of gestation, increases with gestational age, and has the same characteristics throughout gestation. The data show that the lipase in gastric aspirates differs from pancreatic lipase, but closely resembles human and rat lingual lipase. Because the lipase has a low pH optimum and does not require bile salts, it can act in the stomach where it initiates the hydrolysis of dietary fat. We suggest that intragastric lipolysis is probably of major importance in the newborn and especially in the premature infant where it compensates not only for low pancreatic lipase, but in addition, helps to overcome the temporary bile salt deficiency through the formation of amphiphilic reaction products.