Measurement of RNA, DNA, protein and phospholipid synthesis in the neonatal rat pancreas following the oral administration of partially purified soybean trypsin inhibitor demonstrates an enhanced synthesis of all these constituents. Evidence of true hyperplasia in addition to this cellular hypertrophy is provided by an increased mitotic activity in the exocrine pancreas following a wave of enhanced [3H] thymidine incorporation into DNA. Complete inhibition of the stimulated RNA synthesis by low doses of actinomycin D indicates the importance of transcription as a regulatory step in the response of the exocrine pancreas to trophic stimulation by this means. Collateral observations of [3H] thymidine and [14C] orotic acid incorporation into liver DNA and RNA, respectively, fail to demonstrate comparable changes confirming the probable specificity of the trypsin inhibitor induced effect on the exocrine pancreas. It is suggested that the pronounced trophic effect of orally administered soybean trypsin inhibitor in the neonatal rat pancreas may serve as a useful model for the study of regulatory mechanisms of nucleic acid and protein synthesis in the mammalian pancreas.