This report describes some hormonal changes occuring in the rat during the last 3 days of gestation and the first 6 h after surgical deliversy on days 20–22uu ofgestation. In the foetus, the concentration of glucagon increased 2‐fold and that of insulin decreaed from 4.7 to 3.5 ng/ml between days 21 and 22. Consequently the insulin/glucagon molar ratio decreased from 15.6 on day 21 to 5.9 on day 22. After delivery on dasys 21 and 22 of gestation plama glucagon concentration increased 2‐fold and reeached a maximim at 30 min after birth. In addition, the concentration of serum insulin decreased to about 50% of the foetal values by 30 min post partum. Consequentaly the insulin/glucagon ratio decreased to a minimum 30 min after delivery. There was no sinificant change in the concentration of total, that bound to corticosteroid‐binding globulin and free corticosterone in the plasma nor in the concentration of liver corticosterone for 6 h after delivery on day 22 of gestation. Only free plasma corticosterone concentration increased after delivery on day 21 and peaked one hour after birth. The concentration of both pancreatic hommones and corticosterone showed a different development pattern after premauature delivery on day 20 of gestatin. Plasma glucagon increased and plasma insulin decreased for 2 h. Consequentaly the insulin/glucagon ratio decreased from 8.8 at birth to 2.4 at 2 h post partum. The concentration liver corticosterone as well as total, bound and free corticosterone in the plsma incrreased to maximal levels within 1–2 h ofdelivery. The results support the proposal that the relative concentration of insulin and glucagon in the blood regulate the adenosine 3′:5′‐monophosphate concentration and thus enzyme induction in the liver of the perinatal rat. However, the changes in corticosterone concentration wer not consistent with the view that steroids are a primary signal for tyrosine sminotransferase induciton in the newborn rat.