During Bombyx mori metamorphosis, respiration and weight of serical glands, adipose tissue, ovaries and body walls have been studied after explantation. An intrinsic respiration of organs is defined as oxygen consumption of these organs when explanted. The addition of this intrinsic respiratory rate from different explanted organs gives values practically equivalent to nymphal respiration during a part only of pupal life. The whole of the observations leads to the conclusion that U-shaped curve is a complex resultant between fundamentally distinct processes among which: transfers of substances from one organ to another which express the whole of histolysis and histogenesis phenomena in a closed system such as a nymph; active regulative mecanisms of probably neuroendocrine source. © 1969 Springer-Verlag.