The effect of in situ freezing of the urinary bladder on sodium o-phenylphenate (OPP-Na)-induced urinary bladder tumor development was investigated in male F344 rats. Freezing was performed at the start of the experiment by touching the serosal surface of the bladder with a frozen steel rod. As a result, three out of 27 rats (11%) developed bladder tumors within 78 weeks when 0.5% OPP-Na feeding was started 2 weeks after freezing and one out of 27 rats (4%) when the feeding was started 12 weeks after freezing. 0.5% OPP-Na alone did not induce any bladder lesions. In a second experiment, 19 out of 25 rats (76%) developed bladder tumors (carcinomas in 12 rats and papillomas in seven rats) when 2% OPP-Na was administered from 6 weeks after freezing, whereas only one rat (5%) demonstrated a bladder carcinoma in the group given 2% OPP-Na without prior freezing. In neither experiment were tumors induced by freezing alone. Enzyme histochemistry revealed no remarkable changes in enzyme activities of regenerative hyperplasia induced by freezing. The results indicate that in situ freezing of the urinary bladder acts as a trigger of rapid development of OPP-Na-induced rat urinary bladder tumors.