Whole pancreatic blood flow (PBF) and islet blood flow (IBF) were measured in anesthetized Sprague-Dawley rats aged 5, 20, 52, or 104 wk. PBF decreased progressively with age, from approximately 0.55 mL x min-1 x g pancreas-1 in 5- and 20-wk-old rats to 0.30 mL x min-x g pancreas-1 in 52- and 104-wk-old rats. In animals aged 5 wk, the IBF was approximately 25 microL x min-1 x g pancreas-1 but increased to about 60 microL x min-1 x g pancreas-1 (p less than 0.001) in animals aged 20-104 wks. When IBF was expressed as a fraction of PBF, a different pattern was observed. In the 5-wk-old animals only 5% (p less than 0.001) of the PBF flowed through the islets, whereas the rats aged 20 wk had a fractional IBF of about 10%. A significantly larger fraction, of approximately 20%, was diverted through the islets in the 52- and 104-wk-old rats. Glucose administration to animals aged 5, 20, and 52 wk significantly increased both PBF and IBF, but had a more marked effect in the older animals. The fractional IBF was also increased by glucose in the two latter groups, indicating a preferential increase in IBF. In the animals aged 5 wk, however, PBF and IBF increased in concert. We conclude that the previously described age-dependent changes in islet volume and functional capacity of the islet organ is also accompanied by changes in the IBF.