Propoxur produces bladder tumors in rats, but not other species. The hyperplastic and tumorigenic effects do not occur if urinary pH is lowered by administering propoxur in a semi-synthetic diet or co-administering it with ammonium chloride (NH4Cl). We fed propoxur at 8000 p.p.m. in Altromin 1321 diet to male Wistar rats for 4 weeks, with or without NH4Cl as 10,000 p.p.m. of the diet. The urine of rats fed control diet with or without propoxur had a relatively high urinary pH (approximately 8); the addition of NH4Cl lowered the urinary pH by approximately 0.5-1.0 units. There was no evidence of urinary calculi or amorphous precipitate nor was there an increase in microcrystals or formation of different crystals than occur in normal rat urine. Propoxur produced hyperplasia of the urothelium, as observed by light and scanning electron microscopy, and increased the labeling index for proliferating cell nuclear antigen. These effects were significantly inhibited by co-administration with NH4Cl. There was no evidence of urothelial necrosis. Thus, the hyperplasia appears to result from a direct mitogenic effect of propoxur or a metabolite on the urothelium, rather than from toxicity and consequent regeneration. Based on the present study and previous investigations, the urothelial effects of propoxur in the rat are dependent on high urinary pH and high administered doses, factors which need to be incorporated into any mechanistic model for the chemical and into any extrapolation to potential effects in humans.