In　wireless　sensor　and　actor　networks,　when　events　of　interest　are　detected,　the　sensor　nodes　should　transmit　event　reports　to　inform　the　actors.　Due　to　the　mobility　of　actors,　their　current　locations　are　generally　unavailable　to　the　sensor　nodes.　Location　service　is　a　popular　solution　to　solve　this　problem.　However,　the　actor　is　required　to　periodically　update　its　location　to　the　location　servers,　which　may　waste　considerable　energy　in　the　event-driven　scenario.　In　this　paper,　we　propose　an　algorithm　named　trail-based　location　service　(TLS).　The　basic　idea　of　TLS　is　that,　the　actor　can　leave　its　trail　which　is　composed　of　continuous　footprints.　When　a　footprint　is　discovered　during　the　event　report　transmission,　the　event　report　packet　can　be　forwarded　along　the　trail　until　it　reaches　the　actor.　With　the　help　of　trail　information,　TLS　can　prolong　the　period　of　location　update　and　hence　reduce　the　update　overhead　with　guaranteed　delivery.　The　theoretical　analysis　and　simulation　results　are　provided　to　prove　the　efficiency　of　TLS.