Foetal rat pancreatic explants (20-day postcoitum) were grown in organ culture on medium enriched with serum and embryo extract containing various concentrations of corticosterone. Normal pancreatic exocrine morphology was preserved. In addition, media amylase concentration remained high and media insulin was suppressed. This is in sharp contrast to explants incubated on control medium without addition of steroid in which a rapid dissappearance of the acinar component and a selective proliferation of the islet cells was noted. The magnitude of these effects was related to the concentration of the steroid. Corticosterone was effective in preserving pancreatic acinar cells through 8-10 days in vitro. Removal of high levels of corticosterone from the incubation medium after 4 days of culture resulted in a decrease in media amylase and a fall in explant acinar cell mass. The media insulin returned to control levels during the following 4 days of culture. Addition of corticosterone to the media following 4 days of control culture resulted in no increase in media amylase and no statistically significant differences in explant acinar cell mass. Media insulin was decreased from control levels following the additional 4 days of incubation. However, corticosterone, even at a concentration of 10.0 mug/ml, was not effective in depressing insulin secretion as was foetal adrenal co-culture. It is proposed that adrenal corticosteroids are responsible for the maintenance of differentiated acinar cells previously observed in pancreatic adrenal co-culture. This suggests that corticosteroids may play an important role in in vivo pancreatic morphogenesis and cytodifferentiation. In addition, adrenal corticosteroids directly inhibit insulin release from the explant beta cells in vitro.