Inositol lipid hydrolysis was monitored in the human breast cancer cell line MCF-7 in response to various bombesin (BN) and substance P (SP) analogues. Both members of the BN family of peptides, i.e. BN and gastrin-releasing peptide (GRP), stimulated a dose-related increase in total inositol phosphate production, with a similar half-maximal effective dose (ED50) around 1 nm. The BN receptor antagonist [Leu13-Ψ-CH2NH-Leu14]-BN (LLBN) at 1 μm was devoid of agonist activity and displaced the BN dose—response to the right, resulting in a tenfold increase in the ED50 for BN. BN also stimulated a dose-related increase in 45Ca2+ efflux which was also inhibited by LLBN. Two SP analogues [dArg1, d-Pro2, d-Trp7,9,Leu11]-SP and [d-Arg1, d-Phe5, d-Trp7,9,Leu11]-SP ([APheTL]-SP), both antagonized BN-stimulated inositol lipid hydrolysis. [APheTL]-SP (60 and 80 μm) alone also exhibited considerable agonist activity which was not antagonized by LLBN. Indeed, a sub-threshold dose of [APheTL]-SP (40 μm) in the presence of LLBN (10 μm) potentiated the inositol lipid hydrolysis response. BN, GRP, LLBN and [APheTL]-SP all inhibited binding of 125I-labelled GRP to MCF-7 cells, to 50% of that occurring in the absence of the peptides, at concentrations of 150 pm, 150 pm, 150 nm and 600 nm respectively. These data are consistent with the presence of separate but interacting receptors or binding sites for BN and SP analogues, which are coupled to a common signal transduction pathway in human breast cancer cells.