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Abstract: Human β‐cells are functionally mature by the age of 1 year. The timeline and mechanisms of this maturation are unknown owing to the exceptional availability of testable tissue. Here, we report the first in vitro study of insulin secretion by islets from a 5‐day‐old newborn. Glucose was inefficient alone, but induced insulin secretion, which was concentration‐dependent, showed a biphasic time‐course and was of similar magnitude as in infant islets when β‐cell cyclic adenosine monophosphate was raised by forskolin. Tolbutamide alone was effective in low glucose, but its effect was not augmented by high glucose. Metabolic amplification by glucose was thus inoperative, in contrast to amplification by cyclic adenosine monophosphate. Newborn islets showed high basal insulin secretion that could be inhibited by diazoxide or omission of CaCl2. Postnatal acquisition of functional maturity by human β‐cells implicates control of basal secretion and production of metabolic signals able to activate both triggering and amplifying pathways of insulin secretion. [ABSTRACT FROM AUTHOR]