The effect of pregnancy on insulin secretion from isolated perfused rat pancreas was studied. Hypoglycemia and hyperinsulinemia were observed in pregnant rats in contrast to virgin rats. The infusion of 16.7mM glucose significantly augmented insulin secretion in both the first and second phases in pregnant rats compared with virgin rats, whereas insulin secretion was reduced very nearly to the virgin levels at both phases in one-month postpartum rats. The other nutrient tested, arginine (19mM), was a more potent stimulator than glucose. Not only the first phase, but also the second phase was stimulated significantly by arginine in pregnant rats. When one month had passed after delivery, insulin release was diminished to the virgin levels. These data confirm the suggestion that secretory activity is enhanced in the pancreas of pregnant rats.