The discoidal/fusiform vesicles (DFV) of bladder umbrella cells undergo regulated exocytosis in response to stretch, but little is known about their biogenesis or the molecular machinery that modulates this process. We observed that Rab11a was expressed in umbrella cells (but not Rab11b or Rab25) and was associated with DFV. Using adenovirus-mediated delivery we transduced umbrella cells in situ with either dominant active (DA) or dominant negative (DN) mutants of Rab11a. DA-Rab11a stimulated an increase in apical surface area in the absence of stretch, whereas DN-Rab11a inhibited stretch-induced changes. Endocytosed fluid and membrane markers had little access to Rab11a-positive DFV, but virally expressed human growth hormone (hGH), a secretory protein, was packaged into DFV. Whereas expression of DA-Rab11a stimulated release of hGH into the bladder lumen, expression of DN-Rab11a had the opposite effect. Our results indicate that DFV may be biosynthetic in nature and that their exocytosis depends on the activity of the Rab11a GTPase.