Neuropeptide K (NPK) is an N-terminally extended derivative of neurokinin A (NKA) that can be a final product in the posttranslational processing of beta-preprotachykinin. A rat salivation bioassay was used to demonstrate potent effects of NPK at low doses, while effects due to NKA were much weaker at higher doses. The rank order of potency of beta-preprotachykinin-derived peptides on salivation responses was NPK greater than substance P greater than NKA much greater than beta-preprotachykinin-(72-96)-peptide. The time course of the NPK response was longer than that observed with substance P. The responses elicited by NPK were blocked by the tachykinin antagonist [D-Pro2,D-Trp7,9]substance P but not by atropine. In peptide coinfusion studies, NPK strikingly potentiated the salivation responses elicited by substance P. NPK in vitro displayed a 100 times lower potency than substance P in displacing 3H-labeled substance P binding in submandibular gland membranes, a tissue rich in SP-P type (NK-1) receptors. The possible cellular mechanisms by which NPK stimulates salivary gland secretion are discussed. We conclude that NPK and substance P may be cotransmitters derived by posttranslational processing of beta-preprotachykinin.