Annexin A4 (anxA4) is a member of the Ca(2+)-dependent membrane-binding family of proteins implicated in the regulation of ion conductances, Ca(2+) homeostasis, and membrane trafficking. We demonstrate, in mice, that annexins 1-6 are present in whole bladder and exhibit differential expression in the urothelium. An anxA4a-knockout (anxA4a(-/-)) mouse model shows no protein in the urothelium by immunofluorescence and immunoblotting. In wild-type bladders, anxA4a in umbrella cells showed uniform cytoplasmic staining and some association with the nuclear membrane. Application of a hydrostatic pressure to bladders mounted in Ussing chambers resulted in redistribution of anxA4a from cytoplasm to cellular boundaries in the basal and intermediate cells but not in superficial umbrella cells. We hypothesized that anxA4a might be important for barrier function or for stretch-activated membrane trafficking. To test these hypotheses, we conducted a series of functional and morphological analyses on bladders from control and anxA4a(-/-) animals. The transepithelial resistances, water permeabilities, and urea permeabilities of anxA4a(-/-) bladders were not different from controls, indicating that barrier function was intact. Membrane trafficking in response to hydrostatic pressure as measured by capacitance increases was also normal for anxA4a(-/-) bladders. Cystometrograms performed on live animals showed that voiding frequency and intrabladder pressures were also not different. There were no differences in bladder surface morphology or cellular architecture examined by scanning and transmission electron microscopy, respectively. We conclude that loss of anxA4 from the urothelium does not affect barrier function, membrane trafficking, or normal bladder-voiding behavior.