The effect of insect-moulting steroid, ecdysterone, on protein synthesis in mouse liver was studied. The administration of ecdysterone caused an early stimulation of protein synthesis, as indicated by the marked increase in the incorporation of 14C-chlorella hydrolysate into hot-acid insoluble protein, as early as 2 hr after the treatment as well as does 4-chlorotestosterone. Ecdysterone was able to exert its stimulatory effect on the protein synthetic ability of microsomes or polysomes, but not S-105 fluid, soluble components for protein synthesis, except in microsomal system. The stimulation induced by ecdysterone was partly insensitive to actinomycin, inhibitor of DNA-dependent RNA synthesis, but the stimulation by 4-chlorotestosterone was completely repressed by it.