The urinary metabolites from repeated oral doses of 3.7 mg o-phenyl phenol (OPP) to mature and immature dogs and cats were studied. At both age levels, dogs excreted significantly more OPP as sulfate and glucuronide than did cats. Puppies produced 4 times the level of glucuronides than mature dogs. No such age differences were seen with glucuronide formation by cats, nor were there any age differences in either group of animals for sulfate formation. Some sex differences were observed in conjugation of OPP in cats and dogs. The dominant urinary excretion product of oral OPP administration was the unchanged OPP.