It is well known that amino acid sequences of several bioactive peptides are well conserved between mammalians and amphibians [1]. In fact, we have identified in porcine spinal cord a series of neuromedins highly homologous to amphibian peptides [2-4]. Therefore, we began a systematic search for unidentified bioactive peptides in frog (Rana catesbeiana) brain and intestine using a bioassay system for a stimulant effect on guinea pig ileum. Here we report isolation and complete amino acid sequences of four novel bioactive peptides. By structural analyses, these peptides are found to have highly homologous sequences to other known tachykinins. Thus, we designated them ranatachykinin (RTK) A, B, C and D. Up to now, the mammalian tachykinin family has been shown to be composed of three distinct peptides, substance P, neuromedin K and neuromedin L. Neuromedin K is also known as neurokinin B (or β) [2,5], and neurokinin L as substance K or neurokinin A (or α) [3,5,6]. Diversity of tachykinins in a single frog species may suggest the presence of a complex peptidergic regulation system in amphibians.