In Asterina gibbosa regeneration of a cut-off arm tip cannot occur if the radial nerve has been removed. This regeneration needs the presence of the oral nerve ring connected with the radial nerve intact up to the level of the wound tissues. The oral nerve ring can be considered as a central nervous system. Exposure of the radial nerve to irradiation induces a delay in regeneration. These data suggest there is a stimulation from the oral nerve ring up to the wound tissues. When the tissues involved in regeneration, except for the radial nerve, have been destroyed by irradiation, regeneration is inhibited. Stimulation by a healthy radial nerve is necessary to maintain differentiation and growth of the blasternal cells. The amount of blasternal growth and differentiation depends on the time when the radial nerve ceases to stimulate regeneration. The action of the nervous system is needed throughout the whole course of regeneration.