Binding of 125I-labelled peptides, cytoplasmic Ca2+ concentration ([Ca2+]i) and amylase release were studied in guinea-pig pancreatic acinar cells during exposure to substance P (SP), and cholecystokinin octapeptide (CCK-8). Pre-incubation of cells at 22 degrees C with 0.03 nM to 1 microM SP for 10 min or at 37 degrees C for 5 min followed by acid or neutral washes reduced subsequent binding of 125I-Bolton-Hunter reagent-labelled SP (125I-BH-SP) in a biphasic manner by up to 95%. Incubation at 4 degrees C eliminated high-affinity binding of 125I-BH-SP and concentrations of SP above 1 nM were required for inhibition of subsequent tracer binding. Pre-incubation of cells at 37 degrees C with 1 nM to 1 microM CCK-8 for 10 min followed by neutral washes reduced subsequent binding of 125I-BH-CCK-8 by up to 65%. In cell suspensions, the [Ca2+]i response to SP was gradually reduced by pre-exposure to increasing agonist concentrations from 0.2 to 20 nM. Pre-incubation with high SP concentrations for 10 min caused profound reduction of subsequent amylase responses to SP, whereas secretion was little affected in corresponding experiments with CCK-8. Down-regulation of receptor binding is not important during short exposure to CCK-8, but it is a pronounced and rapid phenomenon during SP exposure, which explains tachyphylaxis of [Ca2+]i and amylase responses.