Purpose: Experimental partial bladder outlet obstruction in rats and rabbits drives the bladder through 3 sequential responses, referred to as hypertrophy, compensation and decompensation. The hypertrophy phase, which is a period of rapid bladder growth, has previously been shown to be accompanied by a significant increase in bladder blood flow in rats and rabbits in a manner that likely supports the bladder cell growth process. However, chronic periods of obstruction in the rabbit have been shown to reduce significantly bladder blood flow, especially to the detrusor smooth muscle, corresponding with a loss of bladder contractile function or decompensation in these animals. We determined the effects of chronic 1 to 4-week partial outlet obstruction on rat bladder blood flow and directly correlated them with hypoxia in the rat bladder. Materials and Methods: Rats underwent surgical partial bladder outlet obstruction under anesthesia. At weekly intervals after surgery relative blood flow to the bladder and spleen was measured by a fluorescent microsphere infusion technique. Sham operated rats were also studied 2 and 4 weeks following surgery. In a second experiment groups of similarly obstructed rats were treated with Hypoxyprobe-1 (Natural Pharmacia International, Inc., Research Triangle Park, North Carolina), a chemical probe for hypoxia, 3 days, 1 and 2 weeks after partial bladder outlet obstruction. The bladders were subsequently fixed and immunostained using a monoclonal antibody that detects Hypoxyprobe-1 adducts that are selectively formed in hypoxic cells. Results: Neither bladder weight nor bladder relative blood flow was affected by sham surgery. Likewise, control and sham obstructed rat bladders were found to be free of Hypoxyprobe-1 reactive areas. In contrast, obstructed rats had significantly increased bladder weight at all time points. Relative weight of the obstructed rat bladders indicates the response to mild-moderate obstruction. Bladder relative blood flow in obstructed rats was significantly elevated 1 and 2 weeks after partial bladder outlet obstruction but it returned to almost control levels by 3 and 4 weeks. Hypoxyprobe-1 staining demonstrated a sequential transition of hypoxia from bladder mucosa and submucosal regions at 3 days to muscularis and serosal fibroblasts 1 week and finally to smooth muscle cells by 2 weeks after obstruction. Conclusions: In contrast to the rabbit model, global blood flow in the mild-moderate chronically obstructed rat bladder was found to be higher or nearly equivalent to blood flow in unobstructed control rat bladders. However, even in the presence of normal or above normal blood flow focal regions of hypoxia were still observed in obstructed rat bladders and these regions changed with time. These results provide a reason to understand better why rats are more resistant to the onset of bladder decompensation than rabbits and support the concept that hypoxia is involved in bladder remodeling as well as in progressive functional impairment of the bladder after partial bladder outlet obstruction.