Lipase and trypsin activities were estimated in preprandially aspirated duodenal juice of preterm infants with gestational ages between 29 and 32 weeks (group I, n = 33) or between 33 and 36 weeks (group II, n = 22) during the first 6 weeks of postnatal life. The results were compared with the enzyme activities measured in 2- to 6-year-old children. There were no significant differences of the mean lipase or trypsin activities between the two groups lipase; group I 13.7 +/- 7.9, group II 15.9 +/- 9.8 U/ml; trypsin: group I 7.9 +/- 4.7, group II 8.5 +/- 5.1 U/ml). The activities of both enzymes increased significantly and similarly in both groups with postnatal age (lipase: group I r = 0.732, p < 0.01, group II r = 0.743, p < 0.01; trypsin: group I r = 0.705, p < 0.01, group II r = 0.669, p < 0.01). At the end of the study the lipase activities of both groups reached approximately 35% of the values found in the older children, whereas the trypsin activities reached the reference values within the 1st month of life. The results indicate an asynchronously age-related development of lipase and trypsin. The development of the lipase activity is delayed in comparison to the trypsin activity which should be considered in the nutritional management of preterm infants.