Intravenous injections of either prostaglandin E1 or lysine bradykinin (kallidin) modified the secretory response of the submandibular, parotid and sublingual glands of the rat to intravenous infusions of acetylcholine. The two substances caused reductions (of from 16 to 67 per cent) in salivary flow rates when administered in concentrations ranging from 10 to 50 micrograms/kg body weight. The effect lasted for 20-30 min, followed by a return to pre-injection levels and in general, depended on the concentration of the secretory stimulator and on the dose of test substance used. In the submandibular and sublingual gland, both substances generally caused a concomitant increase in the salivary Na+ concentrations. This effect depended on the concentrations of acetylcholine and of test substance and varied from 10 to 117 per cent. The effect was more marked in submandibular saliva. Absolute increases in salivary Na+ concentration were not observed in the parotid gland, but the reductions in salivary Na+ concentrations (from 2.5 to 31.7 per cent) were smaller and did not parallel the simultaneous reduction in flow rate, which was between 16.4 and 60.4 per cent. As both kinins and prostaglandins are present in the glands and may be activated as a result of secretory or metabolic activity, the results suggest that they act as modulators of the secretory response to cholinergic stimulation. The divergent effects on flow rate and on salivary Na+ concentration suggest that kinins and prostaglandins have specific and independent effects on acinar and duct cells.