Abstract: A nonshrinkable, thermoplastic and injectable endodontic filling composition. The composition is a mixture of transpolyisoprene, either naturally produced as balata or gutta percha, or synthesized from isoprene, and a liquid castor ester platicizer. Alternatively, the mixture comprises transpolyisoprene, a liquid castor ester plasticizer and a liquid castor oil or dimethyl silcon fluid lubricant. The mixture is preferably a combination of the transpolyisoprene, the liquid castor ester plasticizer and the liquid castor oil or liquid dimethyl silicon fluid lubricant or both. The composition is compounded to form a highly thermoplastic material which is injectable at a clinically acceptable temperature with a manually operated injection system without shrinkage of the composition after placement in a dental root canal.