Fetal rat pancreas (ages 16 to 22 days postcoitum) and neonatal pancreas (4 days postnatal) were grown in organ culture for four days. The medium consisted of chick embryo extract and rooster serum either with or without the addition of corticosterone (3 × 10−5 M). Acinar cell differentiation was assessed using quantitative light microscopic linear scanning of tissue sections and enzymatic analysis of amylase in the culture media and in the explants. In the younger fetal tissue of 16 and 18 days postcoitum exocrine differentiation continued in vitro. The effect of corticosterone was an enhancement of the degree of in vitro differentiation. Even with corticosterone, however, the degree of differentiation in vitro was less than that observed during a similar period in vivo. In differentiated pancreas (20- and 22-day fetal and 4-day neonatal) the acinar pancreas degenerated under control conditions and a selective growth of the endocrine pancreas was observed. The addition of corticosterone to the media resulted in a maintenance of the differentiated state of the acini except in 22-day fetal tissue in which the acini were not preserved. The differences between these results and the work of other investigators and the possible in vivo role of adreno corticosteroids in exocrine pancreatic differentiation is discussed.