The sodium ionophore monensin caused a secretory response which was characterized by small volumes of saliva with high K and HCO3 and low Na and Cl concentrations. When added to the perfusate concomitantly with acetylcholine, it reduced the volume of saliva secreted in 60 min by 74 per cent and caused an increase in salivary Na and Cl concentrations at low rates of flow. When glands were exposed to monensin for 30 min prior to acetylcholine stimulus, the fluid secretory response to the secretagogue was inhibited 96 per cent and the electrolyte composition of that secretion had higher Na and Cl and lower K and HCO3 concentrations than the monensin-induced one. The small secretory response induced by monensin is likely to be the result of Na entry into the salivary cells, which is of such magnitude that it rapidly dissipates the transmembrane Na gradient. This may affect the Na, K pump responsible for the maintenance of this gradient and the ion-transport systems dependent on it.