The effect of the diuretic drug furosemide on two‐stage urinary bladder carcinogene‐sis in F344 rats initiated byN‐butyl‐N‐(4‐hydroxybutyl)nitrosamine (BBN) was investigated with regard to possible promoting activity. BBN was administered at 2 doses, 0.01 or 0.05%, in drinking water for 4 wk, and thereafter furosemide was given by gavage 3 times weekly for 32 wk, 250 mg/kg body weight. Furosemide ingestion induced diuresis with an alkaline, hypotonic urine. No significant difference with regard to incidences of bladder lesions were apparent between furosemide and control groups. The present investigation indicated that neither furosemide nor its related polyuria acted as a promoter in two‐stage urinary bladder carcinogenesis.