Strips of rat detrusor muscle were studied in an organ bath 6 months after X-irradiation at doses of 15 and 25 Gy; cystometric studies in these animals had shown a persistent and significant reduction in compliance. The organ bath study demonstrated an increase in the purinergic sensitivity of irradiated detrusor muscle as compared with control. This was significant: p < 0.0145 for the 25 Gy dose group (n = 8) and p < 0.0456 for the 15 Gy group (n = 8) at an alpha,beta-methylene-ATP concentration of 10(-4) M (Mann-Whitney U-Test). There was no difference in sensitivity to cholinergic or noradrenergic stimulation, or to electrical stimulation of the transmural nerves. The finding of purinergic hypersensitivity in irradiated muscle, coupled with ultrastructural evidence of a neural injury, raises the interesting possibility that a denervation supersensitivity phenomenon may contribute to the pathophysiology of post-irradiation bladder dysfunction.