Enzyme assays and morphological and histological studies show that the opaque zone midgut cells of the hematophagous fly Stomoxys calcitrans are responsible for the production of proteolytic digestive enzymes and that these are secreted into the gut lumen via membrane bound vesicles (MBV). The secretory cycle can be summarized as follows: initially the rough endoplasmic reticulum is stacked and the apices of the cells are packed with MBV. This is followed by a period of release characterized first by cytoplasmic extrusions containing high densities of MBV, then by microvesiculation of the microvilli combined with a progressive distribution of rough endoplasmic reticulum and lightening of the cellular cytoplasm. Glycogen appears in the cells at this stage and is gradually lost as the rough endoplasmic reticulum becomes stacked once more and the numbers of MBV build up again. The cycle which occurs regularly and synchronously in the cells of the zone repeats itself many times up to the completion of digestion of the blood meal. The secretory cycle is discussed with reference to activity in other secretory tissues.