In the present study the biotransformation of phenanthrene in the rat and guinea pig was investigated.14C‐labelled phenanthrene was administered by gavage in com oil to Sprague‐Dawley rats (10 mg/kg b.w./day) and guinea pigs (10 mg/kg b.w./day). Urine and feces were separately collected for the determination of the radioactivity content, and pooled urine was used for the analysis of metabolites. Phenanthrene was metabolized by the rat and guinea pig to free hydroxylated phenanthrenes and their conjugates. The percentages of conjugates, expressed as the total urinary radioactivity, were 39% glucuronides, 24% sulfates and 18% cysteinylglycine for rats; and 39% glucuronides, 23% sulfates and 28% cysteinylglycine for guinea pigs. Enzymatic hydrolysis of glucuronides and sulfates resulted in the formation of free 1,2‐, 3,4‐ and 9,10‐dihydrodiols of phenanthrene and 1‐, 2‐, 3‐, and 4‐hydroxyphenanthrene in both species.