Preparations of rat detrusor vesicae urinariae exposed to 50 kV X-irradiation with 10 to 200 Gy (single dose) at dose-rates of 30 and 60 Gy/min reacted immediately with a dose and dose-rate dependent reversible or persistent increase (up to ten hours) of the basal tone and an increase or a decrease of the acetylcholine contractile response. The motor activity was recorded isotonically. For measurements of time changes following treatment in vivo the bladder was locally irradiated from lateral position with single 300 kV X-ray doses of 10, 25 and 50 Gy. The motor reaction of isolated detrusor preparations to acetylcholine had a threshold concentration in control animals of 2.3 X 10(-10) mol/l (n = 33); the sensitivity to acetylcholine was diminished as early as one to two hours after local irradiation with 50 Gy as reflected in a ten times higher threshold concentration, which decreased further with time past treatment up to 40 days. The inhibitory effect after 25 Gy was weaker. The contractile response of acetylcholine at different concentrations (10(-10) to 10(-5) mol/l) was also diminished after irradiation (50 Gy). It is suggested that the pathophysiological reactions of the radiogenic bladder are based on multifactorial mechanisms and X-ray induced tonic contraction as well as inhibition of the acetylcholine contractile response could be essential factors for the clinically observed hypertonia of the irradiated bladder ("radiogene Harnblase") and its functional volume decrease as well as of the diminished pressure during micturition.