Muscles of the ambulacral legs of the starfish A. rubens were sensitive to the effect of ACh acetylcholine reacting to its injection by shortening of the contracture. Amplitude of the contractile response was directly related to ACh concentration. The highest sensitivity was observed during complete stretching of the leg from the body of the starfish, threshold contractions being recorded in response to 1 .cntdot. 10-8 M. During incomplete stretching of the leg, the sensitivity was lower. Carbocholine sensitivity was higher than acetylcholine sensitivity. Proserine increased ACh sensitivity; the increase was very significant (1000 times) provided the leg was completely stretched. Atropine and D-tubocurarine decreased muscle sensitivity to ACh, shifting dose effect curves to the right. Muscles of the ambulacral leg probably possess an ACh-cholinoreceptor system, its concentration being the highest at the base of the leg. Cholinoreception is not limited by M- and H-types only.