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Although vesicles are found in secretory tissues of both invertebrates and vertebrates, their role in fluid transport remains unclear. The Malpighian tubules (Mt) of Acheta domesticus exhibit extensive vacuolation in response to secretagogues. When Mts are treated with cAMP, which causes a doubling in secretion rate, [is greater than] 12% of the cell volume is occupied by vesicles, compared with [is less than] 1% in unstimulated controls. Using Lucifer Yellow CH, a fluid-phase fluorescent marker, we have demonstrated that the extracellular fluid enters the Mt cells via endocytosis. The dye also appears in the secreted urine, showing that it does move completely across the Mt cells. At present we are using electron-dense markers to track vesicle movement in order to show whether the transcellular movement is indeed transcytosis or whether some other mechanism is involved. Supported by USDA-CREEF grant 9501834.