Actinomycin-D was administered to regenerating stem segments ofTubularia spectabilis to ascertain if the synthesis of RNA is necessary for the completion of hydranth development. Preliminary experiments have indicated that this drug suppresses a burst of H3-uridine uptake into the extractable RNA of these regenerates.Wound healing was unaffected by doses of this drug which significantly retarded subsequent hydranth differentiation.The degree of regenerative suppression in this animal increases as a direct function of the administered concentration of actinomycin-D.The period of greatest sensitivity to actinomycin-D occurs early in the regenerative period: hours 6-17 following hydranth amputation. Wittman (1969) has demonstrated a puromyein-sensitive peak of C14-leucine uptake into proteins synthesized at hour 25 or beyond. The authors conclude that the control of hydranth differentiation in regeneratingTubularia resides at the transcriptional level of gene action.