The reactivity of pancreatic B-cells of the decapitated and encephalectomized rat foetuses was determined by changes of insulin secretion in response to glucose introduction in the incubation medium. Upon removal of hypophysis together with hypothalamus in the intact rat females and females with alloxanic diabetes on the 17.5-18.5 day of prenatal development, their pancreas, unlike that of the control foetuses, remained refractory to glucose on the 21.5 day of development. The removal of hypothalamus only upon encephalectomy of the foetuses of the same age led to a similar effect, thus suggesting possible influence of hypothalamus on the regulation of functional maturation of the pancreatic B-cells in the end of prenatal development. Upon the combined incubation of pancreas fragments of the decapitated rat foetuses with adenohypophyses of the adult rats, the effect of decapitation was cut short and the gland responded by the increase of insulin secretion to the rise of glucose content in the medium.