Abstract the present study examines the capacity of larval and pharate pupal wing disks of Samia cynthia ricini to synthesize RNA and proteins and the relation of these syntheses to endogenous and injected juvenile hormone and ecdysone during metamorphosis. The dry weight of the wing disks was found to increase in a linear fashion whereas RNA and protein synthesis showed cyclical patterns. Depending on the stage of development, injection of hormones and their analogues induced changes in the synthetic capacities of the wing disks. An injected hormone had its greatest effect when its endogenous level was lowest and vice versa. At no time were the endogenous titres of juvenile hormone and ecdysone simultaneously high. An antagonistic behaviour was observed when a mixture of two hormones was injected. Although the wing disks do not undergo apolysis and are relieved of the responsibility to synthesize cuticle, their metabolism is affected by injection of the ecdysone. This result suggests a possible rôle for ecdysone besides the induction of moulting.