Prednisolone was administered to two groups of rats for 2 or 4 weeks, respectively. A third group served for control. Serum glucose and insulin levels were measured, and changes in population size along with alterations in staining density of the intracellular granules in pancreatic B-, A- and D-cells were assessed. Two weeks of prednisolone treatment induced significant increases in the staining density of the intracellular granules of B- and D-cells as well as the population size of A-cells, while significant hyperglucosaemia and hyperinsulinaemia were observed. Four weeks of prednisolone administration induced significant increases in population size of B- and D-cells and significant decrease in the population size of A-cells, while hyperglucosaemia and hyperinsulinaemia were even greater. It is concluded that corticosteroids cause an increase in pancreatic B-cell activity and, eventually A-cell numerical atrophy by altering the glucose metabolism. The changes in activity of D-cells possibly reflect functional adaptation to increased B-cell activity.