Nonylphenol is present in surface water and aquatic sediments and because of its lipophilic characteristics shows a considerable potential to bioaccumulate in aquatic organisms. Nonylphenol inhibits testicular growth and induces vitellogenin synthesis in male rainbow trout. In order to better understand the effects of nonylphenol on fish and its impact in the aquatic environment, it is essential to elucidate the metabolic fate of this compound. A single oral dose (5 mg, 1850 KBq) of [3H]4-n-nonylphenol resulted in 1.1% and 3.0% of the ingested radioactivity eliminated in urine after 24 and 48 h, respectively. Four metabolites were separated by radio-HPLC and tentatively identified by mass spectrometry. Urinary metabolites likely resulted from the initial omega-oxidation of 4-n-nonylphenol to the putative 9-(4-hydroxyphenyl)-nonanoic acid which subsequent beta-oxidation led to 4-hydroxybenzoic acid as major metabolite. Intermediary metabolites, namely 3-(4-hydroxyphenyl) propionic acid and 3-(4-hydroxyphenyl)-2-propenoic acid confirmed the occurrence of this beta-oxidative pathway. Urinary metabolites identified in this study were quite different from biotransformation products previously described in bile of trout treated with 4-n-nonylphenol.