The content of ribosomes and the activity of endogenous mRNA increased during adult development in the female pupae of Bombyx mori, but debrained pupae of a strain N122 X C115 did not show such increases. The debrained pupae synthesized ribosomal RNA after injection of ecdysterone. Most of the synthesis was found in the ovaries. The ovaries also exhibited increased activity of endogenous mRNA after injection of the hormone.