Macerated tissues of whole Arenicola marina, when injected into the body cavity of male worms, induce spawning after a prolonged latent period of, generally, 60—90 min. The nature of the spawning reaction is described. Emulsified tissues were submitted to heat, ultra-filtration, extraction by fat solvents and saponification. The technique is described whereby the products of these treatments were assayed for potency in inducing the spawning of male worms. The active substance proved to be a lipid and upon rigorous saponification the fatty-acid fraction was found to induce shedding, but the unsaponifiable fractions failed to do so. The active substance was also found to be a normal tissue constituent of male, female and immature worms, but it is concentrated in the gut and coelomic contents. Tissue extracts appear to induce spawning primarily by the breakdown of the sperm morulae in the coelom to free spermatozoa, but there may also be some modification of the action of the nephromixia. It is suggested that the active substance found in the tissues provides the ‘internal stimulus’ by which spawning is normally induced. Evidence for this suggestion is that it is derived from the tissues of the worm and that it appears to be a specific chemical fraction of the tissues which is effective. In addition, tissue extracts induce a normal spawning reaction and there appears to be a relationship between the temperatures at which the active substance is most potent in inducing shedding in the laboratory and those prevailing on the shore during the breeding season. The possible mode of release of the active substance is discussed.