Modifications in the Malpighian tubules of Rhodnius prolixus during the insect's feeding cycle and under hyperosmotic conditions were analyzed. From the time of one hour after feeding, the proximal region presents a gradual reduction in epithelium volume, with intense vacuolization in the basal region and partial disorganization of the microvilli. Twenty-four hours after feeding many vacuoles without electron-dense content are observed. At 72 hours the volume is large and the mitochondria are found spread throughout the epithelium. The distal region presents concretions of amorphous minerals with an appearance of concentric lamellae. When the tubules are submitted to hyperosmotic shock they present partial disorganization of the microvilli in the proximal region, without alteration of the basal vacuolization. In the distal region, the concretions are expelled into the lumen, together with pieces of membrane, without breaking the integrity of the epithelium, thus maintaining the parallel pattern of the basal interdigitations and suggesting that the ions present are utilized in volume regulation. These data reinforce the importance of feeding in relation to morphological modifications and regulation of cell volume in the luminal and basolateral membrane of the cells of the Malpighian tubules of R. prolixus. In this way, these observations demonstrate that in the physiology of the insect the blood supply is not only related to the nutrient acquisition but to the regulation of the cellular volume and the intense ion exchange due to the osmotic shock that this feeding triggers.