The objective of this study was to investigate the hypoglycemic activity of crude exopolysaccharides (EPS) produced by three mushroom isolates in streptozotocin-induced diabetic rats. The three experimental groups were fed EPS of Cerrena unicolor (Bull.) Murrill (Polyporaceae), Coprinus comatus (O.F. Mull.) Pers. (Agaricaceae), and Lenzites betulina (L.) Fr. (Polyporaceae) for 7 days. The serum glucose levels significantly decreased after oral administration of EPS by 61.23% with Cerrena unicolor, 42.78% with Coprinus comatus, and 42.08% with Lenzites betulina. According to histological observations based on staining in pancreatic tissues, Langerhans islet areas and cell numbers of diabetic animals increased in response to EPS treatment. In conclusion, our findings clearly suggest that exopolysaccharides produced by three mushroom isolates decreased blood glucose levels in STZ-induced diabetic rats. Therefore, the studied mushroom exopolysaccharides might be developed as potential oral hypoglycemic agents in the control of diabetes mellitus. This is the first attempted in vivo study using exopolysaccharides of local mushroom isolates for medicinal purpose in Turkey.