The hibernating eggs of the silkworm turn purple-black by the production of ommochromes within the first four days after the egglaying. The concentrations of xanthommatine and ommine as well as those of tryptophan, kynurenine and 3-hydroxykynurenine were determined at various stages. The levels of tryptophan (maximum at 3.5 days) and of kynurenine (maximum at 1 day after oviposition) were much lower than those of the other substances under consideration. Xanthommatine and ommine begin to develop at the same time after the 3-hydroxykynurenine has reached its maximum value at about 12 hours. The rise of the ommochromes corresponds stoichiometrically to the decrease of 3-hydroxykynurenine.