The dogfish intestinal, linear tachykinin scyliorhinin I (SCYI) and cyclic tachykinin scyliorhinin II (SCYII) bound with differential selectivity to mammalian tachykinin, membrane receptor sites. SCYI bound with highest affinity to NK-1 sites in rat submandibular gland (K I = 0.9 nM) and to NK-2 sites m hamster urinary bladder (K I = 2 nM) whereas SCYII bound with highest affinity to NK-3 sites in rat cerebral cortex (K I = 2.5 nM). These results suggest that SCYI is a dual NK-1/NK-2 tachykinin receptor agonist while SCYII is an NK-3 selective tachykinin receptor agonist.