In order to obtain additional information on serum pancreatic enzyme levels during development, we have measured immunoreactive trypsin (IRT), immunoreactive lipase (IRL), and total amylase in paired fetal and maternal sera. Samples were obtained during early gestation (14-21 week of gestation) and at the time of normal delivery. IRT levels were lower in maternal sera as compared to paired fetal and neonatal (p less than 0.005); conversely, IRL and amylase, although present in measurable concentrations, were significantly lower in fetal and neonatal sera than in the maternal (p less than 0.001). We also serially monitored serum pancreatic enzyme levels in a group of premature infants during the first 10 days of life. Concentrations of IRT showed a significant increase over time (p less than 0.05) and those of IRL remained stable while amylase levels decreased sharply, suggesting possible maternal origin of this enzyme. Serum concentrations of the three pancreatic enzymes in newborns at term (second day of life) were higher than in infants aged 0.5-6 months; however, only IRT levels were above the normal range for adults. Beyond the neonatal period, IRT levels were stable and comparable to adults, whereas amylase and IRL levels were very low in infants younger than 6 months and increased significantly with age (p less than 0.001). These data seem to indicate that "physiologic hypertrypsinemia" occurs early during development and may be accentuated by postnatal events. They provide an indirect indication of both early fetal production of trypsinogen and possible placental transfer of pancreatic enzymes from the maternal circulation.(ABSTRACT TRUNCATED AT 250 WORDS)