The bioluminescent Ca(2+)-sensitive reporter protein, aequorin, was employed to develop an insect cell-based functional assay system for monitoring receptor-mediated changes of intracellular Ca(2)(+)-concentrations. Drosophila Schneider 2 (S2) cells were genetically engineered to stably express both apoaequorin and the insect tachykinin-related peptide receptor, STKR. Lom-TK III, an STKR agonist, was shown to elicit concentration-dependent bioluminescent responses in these S2-STKR-Aeq cells. The EC(50) value for the calcium effect detected by means of aequorin appeared to be nearly identical to the one that was measured by means of Fura-2, a fluorescent Ca(2)(+)-indicator. In addition, this aequorin-based method was also utilised to study receptor antagonists. Experimental analysis of the effects exerted by spantide I, II and III, three potent substance P antagonists, on Lom-TK III-stimulated S2-STKR-Aeq cells showed that these compounds antagonise STKR-mediated responses in a concentration-dependent manner. The rank order of inhibitory potencies was spantide III > spantide II > spantide I