The β-cell function, total islet volume, and number were studied in 1- to 18-month-old mice, together with the extractable pancreatic insulin and glucagon. The β-cell function, determined as the total amount of insulin released in response to glucose from the in vitro perfused pancreas showed an agerelated increase, without any differences in the kinetics of insulin secretion between young and old mice. The total islet number and area in each individual pancreas was determined planimetrically after selective staining of the islets by perfusing the pancreas with dithizone. The islet area increased from 5.4 ± 1.7 mm² at 1 month to 16.3 ± 2.1 mm² at 18 months, whereas the number of islets remained virtually unchanged (1072 ± 51). Pancreatic insulin increased with age by nearly 500%, in contrast to a 35% reduction in pancretic glucagon. There was a strong relationship between body weight and total pancreatic DNA (P = 4.7 × 10^-8), islet area (P = 3.2 × 10^-7), insulin secretory capacity (P = 7 × 10⁴), and total pancreatic insulin (P = 1.9 × 10^-5), but no relationship between body weight and islet number. The insulin secretory capacity increased proportionally to the increase in islet area (P = 9.9 × 10^-3). The islet area and total pancreatic insulin were closely related (P = 2.8 × 10^-12), as were pancreatic insulin and the insulin secretory capacity (P = 3.3 × 10^-11). There was a negative relationship between pancreatic glucagon and islet area (P = 0.005) and between pancreatic glucagon and insulin (P = 0.01). The close relationship between pancreatic insulin and islet area, shown to be an expression of islet volume, makes it possible to estimate the volume of the endocrine pancreas after standard RIA of pancreatic insulin. The combined morphometric and physiological analysis is unique in studying islet cell function relative to the volume of the endocrine pancreas.