Bladder cancer can be induced in the rat by the intravesicular administration of N-methyl-N-nitrosourea. DNA damage in rat bladder epithelial cells after administration of methylnitrosourea has been examined by measuring the change in sedimentation of the DNA in alkaline sucrose gradients. A dose response of DNA damage in the urothelium was observed with single intravesicular doses of 0.1, 0.3, and 0.5 mg of methylnitrosourea. Larger doses of methylnitrosourea damaged the epithelium so extensively, that biochemical studies were not feasible. DNA repair, measured by the return to a normal sedimentation pattern of DNA on alkaline sucrose gradients, was followed over a period of 9 days with the use of 0.5 mg of methylnitrosourea to initiate the damage. Bladder epithelial cells were able to repair the DNA damage induced by methylnitrosourea. However, the possibility of persistent damage not detectable by sedimentation of DNA on alkaline sucrose gradients cannot be ruled out.