Reversed seromuscular flaps of ileum and standard bowel replacement procedures were performed in 16 dogs to evaluate their potential to decrease the likelihood of recognized complications in cases of standard bowel replacement. Of 12 dogs augmentation cystoplasty was done in 6 and ureteral replacement was done in 6. In each group 4 procedures were performed using reversed seromuscular flap, while the remaining 2 were done in the conventional manner (controls). All flap animals had partial to full re-epithelialization with transitional cells but they also had gross and microscopic evidence of flap contraction at the end of 6 months. In the flap augmentation group intravesical pressure measured preoperatively at bladder volumes of 30 cc and 60 cc averaged 25.8 and 45.8 mm. Hg compared to sacrifice pressures of 56.7 and 80.8 mm. Hg. Monthly serum blood urea nitrogen measurements were lower in reversed seromuscular flap animals compared to controls. An additional 4 dogs were studied to help elucidate the etiology of graft contraction, of which 2 underwent reversed seromuscular flap enterocystoplasty with no mucosal stripping while 2 had a procedure exposing intact intestinal serosa to the lumen of the bladder and urine. All of these animals demonstrated good re-epithelialization of the serosal surface with transitional cells as well as little or no evidence of flap fibrosis or contraction. Our results demonstrate that the use of reversed seromuscular flaps in the urinary tract in dogs results in good re-epithelialization of the serosal surface with transitional cells but also flap contraction. This fibrosis and scarring process is largely due to the trauma of mucosal stripping and not urine contact.