The data from the present investigation differ from those of the previous study. The new version of hydrolysed protein formula did not induce changes in insulinaemia or in the insulinaemia/glycaemia ratio in pre- and postprandial samples when compared with the intact protein formula. The investigation also confirmed that branch chain aminoacids regulate insulin secretion and that the length of chain is insulinotropic for fatty acids, while the degree of unsaturation is not able to reduce insulin secretion in the newborn baby. This could be caused by stimulation of insulin output activated by the C20:5 n 3; the effects of fatty acids are probably related more to the variable composition of cell membrane than to the fatty acids circulating levels.