Functional damage in the mouse bladder was measured sequentially from 1 to 53 weeks after irradiation with a range of X ray doses (10 to 30 Gy). Damage was assessed from the independent assays of urination frequency and cystometric measurement of bladder volume at a constant intravesical pressure. There was an early, transient wave of damage from 1 to 3 weeks after bladder irradiation. During this period the urination frequency was increased to greater than or equal to 2 times control levels in 20 to 70% of the mice (depending on dose) after 15 to 30 Gy. Bladder volume was reduced to less than or equal to 50% of control values in 20 to 40% of the mice after doses of 20 to 30 Gy. This early damage usually lasted for less than 1 week and occurred at times ranging from 5 to 21 days, independent of dose. There was no significant correlation between response as measured by the two assays on an individual animal basis during the early period. The incidence of reduced bladder volume, measured cystometrically in anesthetized mice, tended to be less than the incidence of increased urination frequency, measured in non-anesthetized animals. Late bladder damage developed from 16 to 40 weeks after doses of greater than or equal to 20 Gy, and the time of onset was inversely related to dose. Less than 20% of mice treated with 10 to 15 Gy developed late bladder damage as assessed by increased urination frequency or reduced bladder volume. Late bladder damage was irreversible and there was a good correlation between response of individual animals as measured by the two assays. We conclude that changes in both urination frequency and bladder volume can be used as quantitative measures of early and late functional damage after bladder irradiation. The early, transient damage was not associated with changes in the urothelium or muscle layers of the bladder, whereas the late, persistent damage was accompanied by epithelial denudation and focal hyperplasia, with fibrosis and ulceration after higher doses.