The undecapeptide substance P (SP), both activates and 'primes' the neutrophil NADPH-oxidase response. In this paper we investigate the roles of Ca2+ and actin polymerisation in both the activation and 'priming' of the neutrophil oxidase response by the non-oxidisable SP-analog norleucine-SP (n-SP). We demonstrate that by binding to receptors which were distinct from the formylated peptide receptor, n-SP (100 nM-400 microM) directly triggered the NADPH-oxidase response, elevated cytosolic free Ca2+, and caused the polymerisation of G-actin. However at lower concentrations (1-100 nM), in the absence of these phenomena, n-SP primed the oxidase response to the peptide f-Met-Leu-Phe. We propose that occupancy of SP-activatable receptors on neutrophils triggers two different signal transduction pathways, one being responsible for the generation of signals for oxidase activation and the second, which is independent of Ca2+ and actin signalling, being responsible for priming.