Injury to diapausing pupae of the cecropia silkworm moth stimulates the synthesis of RNA in all pupal tissues and the synthesis of several blood proteins; such injury also induces the precocious synthesis of a protein, termed "injury protein," which normally appears in the blood during adult development. Actinomycin D, injected in concentrations of 2 micrograms per gram of body weight, blocks the injury-stimulated increase in blood protein synthesis and the injury-induced synthesis of injury protein. However, at concentrations of 0.5 micrograms per gram it prevents the induction of injury-protein synthesis but does not prevent the increased synthesis of other blood proteins. These results suggest that low concentrations of actinomycin may inhibit the synthesis of new kinds of messenger RNA but still permit the continued synthesis of messenger RNA's already in production at the time the actinomycin is injected.