Perinatal maturation of the pancreatic beta cells was studied in the rat. Insulin synthesis proceeded rapidly be tween the twentieth gestational day and the third postnatal day. The insulin concentration in the pancreas and the amount of insulin available per gram of body weight reached a maximum sixty hours after birth and then decreased slowly toward the values observed in the adult rat. The pancreatic beta cells became responsive to glucose and to tolbutamide during the twenty-third day after fertilization, This phenomenon seemed to be related to the gestational age and not to act of birth or to feeding, although feeding appeared to increase the insulinogenic response to glucose.

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