SUMMARY1. Stimulation of fluid secretion from fly salivary glands by 5-hydroxytryptamine (5-HT) is known to involve calcium and cyclic AMP. Isolated salivary glands were used to investigate the role of these second messengers in the control of enzyme (sucrase) secretion.2. The protein component of secretion from isolated glands treated with 5-HT appears to be identical to that of saliva secreted by flies during feeding.3. Stimulation of fluid secretion by 5-HT follows a definite dose-response curve, but there is no consistent relationship between the rate of enzyme secretion and the stimulating concentration of 5-HT.4. Exogenous cyclic AMP causes secretion of enzymes as well as of fluid, thus mimicking the action of 5-HT. The phosphodiesterase inhibitor theophylline enhances the rate of 5-HT-stimulated enzyme secretion.5. Removal of calcium from the bathing medium enhances enzyme secretion in response to 5 or 10 nM-5-HT but has no effect on enzyme secretion stimulated by 100 nm-5-HT or by cyclic AMP.6. Addition of 0.1 mM-lanthanum to medium containing 2 mM-calcium mimics the effect of calcium-free solution on 5-HT-stimulated enzyme secretion.7. The ionophore A 23187 causes secretion of both fluid and enzyme. The secretary rate is initially high but soon declines and ceases after about 40 min.8. Enzyme secretion in response to 5-HT or to cyclic AMP is progressively inhibited as the concentration of potassium is increased from 10 to 80 mm. Secretion in response to A 23187 is initially inhibited by 80 mm-potassium but then partially recovers.9. The rate of enzyme secretion appears to be affected by the intracellular concentrations of both calcium and cyclic AMP. It is possible that the rate of enzyme secretion increases as the intracellular calcium concentration rises, until the optimal calcium concentration is reached when further increase in the level of calcium progressively inhibits secretion. The optimal calcium concentration for enzyme secretion is lower than that for fluid secretion, and 5-HT normally causes maximal fluid secretion and submaximal enzyme secretion.

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1. A technique has been developed for collecting saliva from the salivary duct of flies during feeding. 2. In newly emerged flies salivation is evoked by stimulation of the labellar taste papillae by crystalline sugar, by sugar in solution and by water. Only sugars which excite the sugar receptors are effective. 3. In flies which have previously fed, solid sucrose or meat induce regurgitation of the crop contents, or salivation if the crop is empty. Sucrose in solution provokes neither reaction. 4. Saliva always contains digestive enzymes, even if the ingested food requires no digestion. 5. Blood taken from salivating flies stimulates fluid secretion from isolated glands, whereas blood taken from non-salivating flies is ineffective. 6. Section of the cephalo-thoracic nerve cord abolishes salivation in response to feeding. Section of the ventral nerve cord posterior to the thoracic ganglion, or removal of the abdomen, severely reduces but does not abolish the salivatory response. 7. Saliva secreted by mature flies contains 75 mM-Cl− whereas saliva secreted during the first meal after emergence has a higher Cl− concentration (about 165 mM-Cl−) which gradually declines during the first meal.