In Tenebrio and Drosophila, as in Calliphora, there are two main pathways of tyrosine metabolism: firstly the hydroxylation to Dopa and the synthesis of N-acetyl-dopamine and secondly the degradation to phenol carboxylic acids. The analogy with sclerotising metabolism in Calliphora and the appearance of N-acetyl-dopamine at the time of sclerotisation suggest a mechanism similar to that found in the cuticle of Calliphora. Maximum activities of Dopa decarboxylase in Tenebrio are also found before pupation and at the beginning of the imaginal stage. The curve of transaminase activity follows the opposite course. In Drosophila, the decarboxylase activity can be shown in vitro too. These findings are discussed in relation to results with other insects.