Substance P, forskolin and isoprenaline stimulated rat parotid alpha-amylase secretion in vitro. Synergistic responses were observed with combinations of any two of the three secretagogues such that subthreshold doses of one caused a pronounced shift in the dose-response curve to the second. This potentiation of secretion could neither be explained by an interaction at the receptor recognition binding site, as identified by ligand binding, nor wholly by interactions in second messenger systems. Thus forskolin and isoprenaline were unable to alter substance P-induced changes in phosphatidylinositol metabolism. Similarly substance P was without effect on forskolin or isoprenaline-stimulated cAMP production. In contrast the potentiation of isoprenaline-induced amylase secretion by forskolin was preceded by a marked enhancement of cAMP production suggesting that the activation of the adenylate cyclase complex is reflected in the cellular response.