Functional restoration of the rat urinary bladder following subtotal cystectomy was studied via in vivo infusion cystometry and an in vitro whole bladder model. After the bladder had been separated from the prostate, subtotal cystectomy was achieved by ligating the bladder completely at a level just above the insertion of the ureters into the bladder. Bladder function was investigated immediately and 7, 14, and 28 days after surgery. Bladder weight was reduced to 17% that in sham-operated controls immediately after surgery, but recovered to 76% of that in controls 28 days after the operation. In vivo capacity also increased after surgery from 13% that of controls to 59% 28 days later. However, voiding pressure remained low (34% of control) 28 days later. An in vitro whole bladder study showed that the response to field stimulation decreased significantly on day 7, but had recovered considerably by day 28. The maximal response to bethanechol decreased significantly 7 days after surgery, but recovered thereafter. The response to phenylephrine increased significantly immediately after surgery, but gradually returned to the control level. An in vitro volume-pressure study showed that passive compliance of the cystectomized bladder decreased after surgery, but improved with time. The peak of the active pressure increase to field stimulation occurred at a low infusion volume immediately after surgery, but bladder capacity increased gradually until 28 days later, when the maximal active pressure was obtained. Our results suggest that restoration of the bladder following subtotal cystectomy may not derive simply from an expansion of the bladder wall. Functional alteration involving the bladder base was also observed.