The effects of an alkalizer or an acidifier on bladder carcinogenesis induced by OPP or OPP-Na were examined. Groups of 31 male F344/DuCrj rats were given 1.25% OPP in the diet (group 1), 1.25% OPP in the diet plus 0.4% sodium bicarbonate (NaHCO3) in drinking-water (group 2), 2% OPP-Na in the diet (group 3) or 2% OPP-Na in the diet plus 1% ammonium chloride (NH4Cl) in drinking-water (group 4) for 26 wk. Urinary bladder tumours, papilloma and carcinoma occurred in 12/31 (39%) of group 1, 20/31 (65%) of group 2, 22/31 (71%) of group 3 and 3/31 (10%) of group 4. Values of urinary pH after 25 wk of treatment were correlated with the tumour incidences. The results indicate that the administration of an alkalizer enhanced the carcinogenicity of OPP and the administration of an acidifer inhibited the carcinogenicity of OPP-Na to the rat urinary bladder. This suggests that the earlier finding that OPP-Na was more carcinogenic than OPP resulted from the higher alkalinity of OPP-Na.