<p>DH31 and drosokinin are known promoters of fluid secretion in <em>Drosophila</em> Malpighian tubules, while the effect of allatostatin A on Malpighian tubule fluid secretion is unknown. The expression of these peptides and their receptors is widespread in the larval gut and central nervous system. The scanning ion-selective electrode technique (SIET) was used to measure changes in epithelial K⁺ flux in the gut as a proxy for the flow of osmotically-obliged water between the gut lumen and the haemolymph. The primary goal of this study was to investigate the effects of DH31 and drosokinin on the gut as an indicator of coordination of diuretic activity between this tissue and the Malpighian tubules. Such coordination, whereby Malpighian tubule fluid secretion is stimulated concomitantly with fluid uptake by the gut, would be physiologically essential for the maintenance of haemolymph volume and osmolarity. Secondarily, this study sought to characterize the function of allatostatin A with respect to its effect on gut K⁺ transport. DH31 stimulated K⁺ absorption across the anterior midgut epithelium and reduced K⁺ absorption in the middle midgut copper cell zone. Drosokinin increased K⁺ absorption across the anterior midgut epithelium and was also stimulatory in the posterior midgut neutral zone. Allatostatin A stimulated K⁺ absorption across the epithelia of the anterior midgut and middle midgut copper cell zone, but was inhibitory in the large flat cell zone. The larger surface area of the anterior midgut relative to the middle midgut means that all three peptides were likely net stimulators of K⁺ uptake. The reduction in K⁺ absorption in the middle midgut by DH31 and allatostatin A may relate to a redistribution of fluid uptake among the regions of the midgut to preserve lumenal pH. DH31 and drosokinin also independently increased the contraction frequency of the anterior midgut, while the contraction frequency of the pyloric sphincter was increased by combined application of both peptides. Stimulation of gut contractions has previously been attributed to these and other diuretic factors in insects. The findings of this investigation implicate DH31, drosokinin, and allatostatin A in the stimulation of midgut K⁺ absorption, which may suggest a role for these peptides in altering fluid transport across this epithelium in larval <em>Drosophila</em>.</p> / Master of Science (MSc)