The effect of single-dose irradiation (19 Gy) or sham treatment on circadian variations in urinary bladder storage capacity was assessed during a 250-day period following treatment using transurethral cystometry under anesthesia. Changes in bladder function were quantified as the deviation from individual pretreatment values of bladder volume at defined intravesical pressures. A marked diurnal rhythm with a peak at 19.00 h and an extended nadir between 1.00 h and 12.00 h was found in 11- to 12-week-old untreated mice. No systematic changes in bladder function with age could be observed in repeated cystometries after sham treatment. X-Irradiation resulted in a triphasic response. After an early reversible decrease in bladder capacity (acute phase) complete recovery was observed within 30 days after irradiation. Following a symptom-free latent period of about 15 weeks, chronic progressive impairment of bladder storage function developed (chronic phase). During the acute and latent period, no changes in the diurnal pattern were observed. In the late phase, not only a decrease in the absolute capacity values but also in the amplitude of diurnal fluctuations was seen. The daily minimum values, however, were still found between 1.00 and 12.00 h. The data indicate that cystometry values obtained between 8.00 and 12.00 h provide suitable parameters for the quantification of radiation effects in the urinary bladder in longitudinal studies.