Distinct chemical cues acting on chemosensory structures on the dorsal lip of bloodsucking leeches activate the entire complement of appetitive and ingestive feeding behaviors. However, it is not known whether the maintenance of ingestion depends on continuous stimulation of these peripheral chemosensors. Leeches of the species Hirudo medicinalis and Macrobdella decora were fed for 2 min on an artificial blood solution containing 150 mM NaCl/1 mM arginine before switching the feeding solution to various experimental mixtures. Leeches did not start to feed on, but continued to ingest solutions in which equiosmolar KCl or lysine substituted for NaCl or arginine, respectively, until sated. In contrast, they rejected water and dropped off the feeding apparatus within 30 s of the exchange. Leeches also detached from the feeding tube when quinine, denatonium, or caffeine were added to the NaCl/arginine solution during an ongoing feeding bout. The duration of ingestion following the switch correlated inversely with the concentration of the drugs (0.1-10 mM). Superfusion of the dorsal lip with high concentrations of the bitter chemicals, while feeding was in progress, had no effect on the duration of ingestion. However, injections of the bitter substances directly into the gut, during a feeding bout, abruptly stopped ingestion. The results suggest that while leeches continue to sample their food once ingestion has begun, secondary chemosensory mechanisms situated downstream from the dorsal lip may be involved in the maintenance of ingestion and the rapid postingestive rejection of noxious foods.