The dart apparatus, used during courtship in some groups of hermaphroditic land snails, has long been assumed to have a "stimulatory" effect on the mating partner, though how stimulation occurs and exactly what function it serves has never been determined. In this study, extracts of the mucous glands of the dart apparatus of the land snail Helix aspersa were injected into conspecifics and into a related snail, Cepaea nemoralis, in order to test the hypothesis that the dart is used to achieve inflow of bioactive mucous gland secretions into the darted snail. Helix aspersa injected with the extract responded by everting their terminal genitals; eversion normally takes place during courtship and mating. Boiling the extract increased the bioactivity. Pronase-treated extract lost bioactivity, and gel filtration of the boiled extract indicated that the active substance has a molecular weight of about 5,000. The active substance may be a polypeptide. Cepaea nemoralis also everted their genitals when injected with the boiled Helix extract. The active substance appears to be a contact sex pheromone, the second such pheromone in a pulmonate land snail for which experimental evidence has been obtained.