Purpose We investigated bladder function, with special focus on initial functional changes, by objective report of decompensated bladder according to the percentage of residual urine volume to bladder capacity in awake, obstructed rats. Materials and Methods Thirty rats were randomly subjected to sham operations (n=10) or partial bladder outlet obstruction (BOO, n=20). Cystometric investigations were performed without anesthesia 1 or 2 weeks after BOO surgery. To reduce the influence of confounding factors in awake cystometry, we used simultaneous recordings of intravesical and intraabdominal pressures. Decompensated bladder was defined as the bladder with more than 20% of residual volume compared with bladder capacity. Results Compared with that in sham animals, basal pressure was elevated in both BOO groups. Threshold pressure was higher in the 2 week BOO (p<0.01) group. Compliance was decreased in the 1 week BOO group (p<0.01) and increased in the 2 week BOO group (p<0.001). Bladder capacity was not increased in the 1 week BOO group, but was increased in the 2 week BOO group (p<0.01). Decompensation was found in 62.5% of the 1 week BOO group and in 33.3% of the 2 week BOO group. Conclusions From the earlier phase, the bladders exhibited serial changes in pressure and volume parameters, and decompensated bladders defined by the percentage of residual volume to bladder capacity could be seen. During the later phase, there was an increasing tendency of compensated bladders, accompanied by the bladders being enlarged and more compliant.