The effects of sodium o-phenylphenate (Na-OPP) treatment on urinary bladder epithelium were examined in male F344 rats, B6C3F1 mice, Syrian golden hamsters and Hartley guinea pigs. Na-OPP was incorporated into diet at a dose of 2% and administered for 4, 8, 12, 24, 36 or 48 weeks. Simple and papillary or nodular (PN) hyperplasias were evident on light microscopy and pleomorphic microvilli demonstrated by scanning electron microscopy were only observed in rats, the lesions becoming more advanced with continued chemical feeding. In mice, hamsters and guinea pigs, proliferative lesions relating to Na-OPP administration were not observed. No significant differences in urinary pH, osmolality or crystal formation were apparent between the various animal species. Since carcinogenicity has been demonstrated for Na-OPP in rats but not in mice, the present findings suggest that Na-OPP might not exert urinary bladder carcinogenic potential in hamsters and guinea pigs.