Ommochrome synthesis at the beginning of lepidopteran metamorphosis is correlated to morpholytic and morphogenetic activity. In order to examine the metabolic alterations involved, data on respiration, haemolymph volume and on concentrations of total carbohydrate, soluble protein and non-protein nitrogen have been collected. Free aminoacids were separated by ion exchange chromatography and the levels of tryptophan, 3-hydroxy-kynurenine and of total ommochromes were determined independently at different stages during transformation of the larva towards the pupa. While the levels of non-protein nitrogen and of total free amino acids remain nearly constant, there is a three- and twentyfold increase of tryptophan and 3-hydroxy-kynurenine, respectively. It is proposed, that ommochrome formation serves in the removal of tryptophan liberated by proteolysis.